diff --git a/core/includes/common.inc b/core/includes/common.inc
index a8ad43a..708bbc9 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -3647,7 +3647,7 @@ function drupal_aggregate_js(&$js_groups) {
  * @code
  * $build['#attached'] = array(
  *   'library' => array(array('taxonomy', 'taxonomy')),
- *   'css' => array(drupal_get_path('module', 'taxonomy') . '/taxonomy.css'),
+ *   'css' => array(drupal_get_path('module', 'taxonomy') . '/css/taxonomy.module.css'),
  * );
  * @endcode
  *
diff --git a/core/includes/theme.maintenance.inc b/core/includes/theme.maintenance.inc
index 70347c2..b9a513c 100644
--- a/core/includes/theme.maintenance.inc
+++ b/core/includes/theme.maintenance.inc
@@ -95,10 +95,10 @@ function _drupal_maintenance_theme() {
   // These are usually added from system_init() -except maintenance.css.
   // When the database is inactive it's not called so we add it here.
   $path = drupal_get_path('module', 'system');
-  drupal_add_css($path . '/system.base.css');
-  drupal_add_css($path . '/system.admin.css');
-  drupal_add_css($path . '/system.theme.css');
-  drupal_add_css($path . '/system.maintenance.css');
+  drupal_add_css($path . '/css/system.module.css');
+  drupal_add_css($path . '/css/system.theme.css');
+  drupal_add_css($path . '/css/system.admin.css');
+  drupal_add_css($path . '/css/system.maintenance.css');
 }
 
 /**
diff --git a/core/misc/dropbutton/dropbutton.base-rtl.css b/core/misc/dropbutton/dropbutton-rtl.css
similarity index 100%
rename from core/misc/dropbutton/dropbutton.base-rtl.css
rename to core/misc/dropbutton/dropbutton-rtl.css
diff --git a/core/misc/dropbutton/dropbutton.base.css b/core/misc/dropbutton/dropbutton.css
similarity index 100%
rename from core/misc/dropbutton/dropbutton.base.css
rename to core/misc/dropbutton/dropbutton.css
diff --git a/core/modules/block/block.admin.inc b/core/modules/block/block.admin.inc
index 10927bd..953e462 100644
--- a/core/modules/block/block.admin.inc
+++ b/core/modules/block/block.admin.inc
@@ -16,7 +16,7 @@
 function block_admin_demo($theme = NULL) {
   return array(
     '#attached' => array(
-      'css' => array(drupal_get_path('module', 'block') . '/block.admin.css'),
+      'css' => array(drupal_get_path('module', 'block') . '/css/block.admin.css'),
     ),
   );
 }
@@ -86,4 +86,3 @@ function block_admin_edit(Block $entity) {
 
   return entity_get_form($entity);
 }
-
diff --git a/core/modules/block/block.admin.css b/core/modules/block/css/block.admin.css
similarity index 100%
rename from core/modules/block/block.admin.css
rename to core/modules/block/css/block.admin.css
diff --git a/core/modules/block/lib/Drupal/block/BlockListController.php b/core/modules/block/lib/Drupal/block/BlockListController.php
index f940b06..9b8be71 100644
--- a/core/modules/block/lib/Drupal/block/BlockListController.php
+++ b/core/modules/block/lib/Drupal/block/BlockListController.php
@@ -106,7 +106,7 @@ public function getFormID() {
    */
   public function buildForm(array $form, array &$form_state) {
     $entities = $this->load();
-    $form['#attached']['css'][] = drupal_get_path('module', 'block') . '/block.admin.css';
+    $form['#attached']['css'][] = drupal_get_path('module', 'block') . '/css/block.admin.css';
     $form['#attached']['library'][] = array('system', 'drupal.tableheader');
     $form['#attached']['library'][] = array('block', 'drupal.block');
 
diff --git a/core/modules/book/book.module b/core/modules/book/book.module
index 61e3701..e5aeafb 100644
--- a/core/modules/book/book.module
+++ b/core/modules/book/book.module
@@ -322,7 +322,7 @@ function book_form_node_form_alter(&$form, &$form_state, $form_id) {
       '#submit' => array('book_pick_book_nojs_submit'),
       '#weight' => 20,
       '#attached' => array(
-        'css' => array(drupal_get_path('module', 'book') . '/book.admin.css'),
+        'css' => array(drupal_get_path('module', 'book') . '/css/book.admin.css'),
       ),
     );
   }
@@ -986,7 +986,7 @@ function template_preprocess_book_all_books_block(&$variables) {
  *     Properties used: bid, link_title, depth, plid, mlid.
  */
 function template_preprocess_book_navigation(&$variables) {
-  drupal_add_css(drupal_get_path('module', 'book') . '/book.theme.css');
+  drupal_add_css(drupal_get_path('module', 'book') . '/css/book.theme.css');
   $book_link = $variables['book_link'];
 
   // Provide extra variables for themers. Not needed by default.
diff --git a/core/modules/book/book.admin.css b/core/modules/book/css/book.admin.css
similarity index 100%
rename from core/modules/book/book.admin.css
rename to core/modules/book/css/book.admin.css
diff --git a/core/modules/book/book.theme-rtl.css b/core/modules/book/css/book.theme-rtl.css
similarity index 100%
rename from core/modules/book/book.theme-rtl.css
rename to core/modules/book/css/book.theme-rtl.css
diff --git a/core/modules/book/book.theme.css b/core/modules/book/css/book.theme.css
similarity index 100%
rename from core/modules/book/book.theme.css
rename to core/modules/book/css/book.theme.css
diff --git a/core/modules/color/color.module b/core/modules/color/color.module
index 2203133..6942c6f 100644
--- a/core/modules/color/color.module
+++ b/core/modules/color/color.module
@@ -194,7 +194,7 @@ function color_scheme_form($complete_form, &$form_state, $theme) {
       ),
       // Add custom CSS.
       'css' => array(
-        $base . '/color.admin.css' => array(),
+        $base . '/css/color.admin.css' => array(),
       ),
       // Add custom JavaScript.
       'js' => array(
diff --git a/core/modules/color/color.admin-rtl.css b/core/modules/color/css/color.admin-rtl.css
similarity index 92%
rename from core/modules/color/color.admin-rtl.css
rename to core/modules/color/css/color.admin-rtl.css
index d5bfbe8..f8dbbd5 100644
--- a/core/modules/color/color.admin-rtl.css
+++ b/core/modules/color/css/color.admin-rtl.css
@@ -31,7 +31,7 @@
 #palette .down,
 #palette .up,
 #palette .both {
-  background: url(images/hook-rtl.png) no-repeat 0 0;
+  background: url(../images/hook-rtl.png) no-repeat 0 0;
 }
 #palette .up {
   background-position: 0 -27px;
diff --git a/core/modules/color/color.admin.css b/core/modules/color/css/color.admin.css
similarity index 91%
rename from core/modules/color/color.admin.css
rename to core/modules/color/css/color.admin.css
index e9aee91..98e9cc2 100644
--- a/core/modules/color/color.admin.css
+++ b/core/modules/color/css/color.admin.css
@@ -45,7 +45,7 @@
 #palette .down,
 #palette .up,
 #palette .both {
-  background: url(images/hook.png) no-repeat 100% 0; /* LTR */
+  background: url(../images/hook.png) no-repeat 100% 0; /* LTR */
 }
 #palette .up {
   background-position: 100% -27px; /* LTR */
@@ -61,7 +61,7 @@
   left: -10px; /* LTR */
   width: 20px;
   height: 25px;
-  background: url(images/lock.png) no-repeat 50% 2px;
+  background: url(../images/lock.png) no-repeat 50% 2px;
   cursor: pointer;
 }
 #palette .unlocked {
diff --git a/core/modules/comment/comment.theme-rtl.css b/core/modules/comment/css/comment.theme-rtl.css
similarity index 100%
rename from core/modules/comment/comment.theme-rtl.css
rename to core/modules/comment/css/comment.theme-rtl.css
diff --git a/core/modules/comment/comment.theme.css b/core/modules/comment/css/comment.theme.css
similarity index 100%
rename from core/modules/comment/comment.theme.css
rename to core/modules/comment/css/comment.theme.css
diff --git a/core/modules/comment/lib/Drupal/comment/CommentRenderController.php b/core/modules/comment/lib/Drupal/comment/CommentRenderController.php
index dab8dc4..c6b21f0 100644
--- a/core/modules/comment/lib/Drupal/comment/CommentRenderController.php
+++ b/core/modules/comment/lib/Drupal/comment/CommentRenderController.php
@@ -86,7 +86,7 @@ protected function alterBuild(array &$build, EntityInterface $comment, EntityDis
 
       // Add indentation div or close open divs as needed.
       if ($is_threaded) {
-        $build['#attached']['css'][] = drupal_get_path('module', 'comment') . '/comment.theme.css';
+        $build['#attached']['css'][] = drupal_get_path('module', 'comment') . '/css/comment.theme.css';
         $prefix .= $comment->divs <= 0 ? str_repeat('</div>', abs($comment->divs)) : "\n" . '<div class="indented">';
       }
 
diff --git a/core/modules/config/lib/Drupal/config/Controller/ConfigController.php b/core/modules/config/lib/Drupal/config/Controller/ConfigController.php
index 477268c..9511a1e 100644
--- a/core/modules/config/lib/Drupal/config/Controller/ConfigController.php
+++ b/core/modules/config/lib/Drupal/config/Controller/ConfigController.php
@@ -63,7 +63,7 @@ public function __construct(StorageInterface $target_storage, StorageInterface $
    */
   public function diff($config_file) {
     // Add the CSS for the inline diff.
-    $output['#attached']['css'][] = drupal_get_path('module', 'system') . '/system.diff.css';
+    $output['#attached']['css'][] = drupal_get_path('module', 'system') . '/css/system.diff.css';
 
     $diff = config_diff($this->targetStorage, $this->sourceStorage, $config_file);
     $formatter = new \DrupalDiffFormatter();
diff --git a/core/modules/contextual/contextual.module b/core/modules/contextual/contextual.module
index 019efed..32f3870 100644
--- a/core/modules/contextual/contextual.module
+++ b/core/modules/contextual/contextual.module
@@ -112,8 +112,8 @@ function contextual_library_info() {
       $path . '/contextual.js' => array('group' => JS_LIBRARY, 'weight' => -2),
     ),
     'css' => array(
-      $path . '/contextual.base.css' => array(),
-      $path . '/contextual.theme.css' => array(),
+      $path . '/css/contextual.module.css' => array(),
+      $path . '/css/contextual.theme.css' => array(),
     ),
     'dependencies' => array(
       array('system', 'jquery'),
@@ -133,7 +133,7 @@ function contextual_library_info() {
       $path . '/contextual.toolbar.js' => array('group' => JS_LIBRARY, 'weight' => -1),
     ),
     'css' => array(
-      $path . '/contextual.toolbar.css' => array(),
+      $path . '/css/contextual.toolbar.css' => array(),
     ),
     'dependencies' => array(
       array('system', 'jquery'),
diff --git a/core/modules/contextual/contextual.base.css b/core/modules/contextual/css/contextual.module.css
similarity index 100%
rename from core/modules/contextual/contextual.base.css
rename to core/modules/contextual/css/contextual.module.css
diff --git a/core/modules/contextual/contextual.theme-rtl.css b/core/modules/contextual/css/contextual.theme-rtl.css
similarity index 100%
rename from core/modules/contextual/contextual.theme-rtl.css
rename to core/modules/contextual/css/contextual.theme-rtl.css
diff --git a/core/modules/contextual/contextual.theme.css b/core/modules/contextual/css/contextual.theme.css
similarity index 98%
rename from core/modules/contextual/contextual.theme.css
rename to core/modules/contextual/css/contextual.theme.css
index fef8240..7a34421 100644
--- a/core/modules/contextual/contextual.theme.css
+++ b/core/modules/contextual/css/contextual.theme.css
@@ -27,7 +27,7 @@
 .contextual .trigger {
   background-attachment: scroll;
   background-color: #fff;
-  background-image: url("../../misc/edit.png");
+  background-image: url(../../../misc/edit.png);
   background-position: center center;
   background-repeat: no-repeat;
   background-size: 16px 16px;
diff --git a/core/modules/contextual/contextual.toolbar-rtl.css b/core/modules/contextual/css/contextual.toolbar-rtl.css
similarity index 100%
rename from core/modules/contextual/contextual.toolbar-rtl.css
rename to core/modules/contextual/css/contextual.toolbar-rtl.css
diff --git a/core/modules/contextual/contextual.toolbar.css b/core/modules/contextual/css/contextual.toolbar.css
similarity index 90%
rename from core/modules/contextual/contextual.toolbar.css
rename to core/modules/contextual/css/contextual.toolbar.css
index 189a07f..69fc317 100644
--- a/core/modules/contextual/contextual.toolbar.css
+++ b/core/modules/contextual/css/contextual.toolbar.css
@@ -5,11 +5,11 @@
 
 /* Tab icon. */
 .icon-edit:before {
-  background-image: url("../../misc/edit.png");
+  background-image: url(../../../misc/edit.png);
 }
 .icon-edit:active:before,
 .active.icon-edit:before {
-  background-image: url("../../misc/edit-active.png");
+  background-image: url(../../../misc/edit-active.png);
 }
 
 /* Tab appearance. */
diff --git a/core/modules/dblog/dblog-rtl.css b/core/modules/dblog/css/dblog.module-rtl.css
similarity index 100%
rename from core/modules/dblog/dblog-rtl.css
rename to core/modules/dblog/css/dblog.module-rtl.css
diff --git a/core/modules/dblog/dblog.css b/core/modules/dblog/css/dblog.module.css
similarity index 87%
rename from core/modules/dblog/dblog.css
rename to core/modules/dblog/css/dblog.module.css
index 5494553..5732388 100644
--- a/core/modules/dblog/dblog.css
+++ b/core/modules/dblog/css/dblog.module.css
@@ -37,11 +37,11 @@
   width: 16px;
 }
 .admin-dblog .dblog-warning .icon {
-  background-image: url(../../misc/message-16-warning.png);
+  background-image: url(../../../misc/message-16-warning.png);
 }
 .admin-dblog .dblog-error .icon,
 .admin-dblog .dblog-critical .icon,
 .admin-dblog .dblog-alert .icon,
 .admin-dblog .dblog-emergency .icon {
-  background-image: url(../../misc/message-16-error.png);
+  background-image: url(../../../misc/message-16-error.png);
 }
diff --git a/core/modules/dblog/dblog.module b/core/modules/dblog/dblog.module
index 63e2b8f..202cfcf 100644
--- a/core/modules/dblog/dblog.module
+++ b/core/modules/dblog/dblog.module
@@ -88,7 +88,7 @@ function dblog_menu() {
  */
 function dblog_page_build(&$page) {
   if (arg(0) == 'admin' && arg(1) == 'reports') {
-    $page['#attached']['css'][] = drupal_get_path('module', 'dblog') . '/dblog.css';
+    $page['#attached']['css'][] = drupal_get_path('module', 'dblog') . '/css/dblog.module.css';
   }
 }
 
diff --git a/core/modules/edit/css/edit.css b/core/modules/edit/css/edit.module.css
similarity index 100%
rename from core/modules/edit/css/edit.css
rename to core/modules/edit/css/edit.module.css
diff --git a/core/modules/edit/edit.module b/core/modules/edit/edit.module
index c29672e..4353ddf 100644
--- a/core/modules/edit/edit.module
+++ b/core/modules/edit/edit.module
@@ -90,7 +90,7 @@ function edit_library_info() {
       $path . '/js/theme.js' => $options,
     ),
     'css' => array(
-      $path . '/css/edit.css' => array(),
+      $path . '/css/edit.module.css' => array(),
     ),
     'dependencies' => array(
       array('system', 'jquery'),
diff --git a/core/modules/entity_reference/css/entity_reference-rtl.admin.css b/core/modules/entity_reference/css/entity_reference.admin-rtl.css
similarity index 100%
rename from core/modules/entity_reference/css/entity_reference-rtl.admin.css
rename to core/modules/entity_reference/css/entity_reference.admin-rtl.css
diff --git a/core/modules/field/theme/field-rtl.css b/core/modules/field/css/field.module-rtl.css
similarity index 100%
rename from core/modules/field/theme/field-rtl.css
rename to core/modules/field/css/field.module-rtl.css
diff --git a/core/modules/field/theme/field.css b/core/modules/field/css/field.module.css
similarity index 100%
rename from core/modules/field/theme/field.css
rename to core/modules/field/css/field.module.css
diff --git a/core/modules/field/field.module b/core/modules/field/field.module
index 5b762b5..ec196e9 100644
--- a/core/modules/field/field.module
+++ b/core/modules/field/field.module
@@ -982,7 +982,7 @@ function field_extract_bundle($entity_type, $bundle) {
  */
 function field_page_build(&$page) {
   $path = drupal_get_path('module', 'field');
-  $page['#attached']['css'][$path . '/theme/field.css'] = array('every_page' => TRUE);
+  $page['#attached']['css'][$path . '/css/field.module.css'] = array('every_page' => TRUE);
 }
 
 /**
@@ -1034,7 +1034,7 @@ function template_preprocess_field(&$variables, $hook) {
     'field-label-' . $element['#label_display'],
   );
   // Add a "clearfix" class to the wrapper since we float the label and the
-  // field items in field.css if the label is inline.
+  // field items in field.module.css if the label is inline.
   if ($element['#label_display'] == 'inline') {
     $variables['attributes']['class'][] = 'clearfix';
   }
diff --git a/core/modules/field_ui/field_ui.admin-rtl.css b/core/modules/field_ui/css/field_ui.admin-rtl.css
similarity index 100%
rename from core/modules/field_ui/field_ui.admin-rtl.css
rename to core/modules/field_ui/css/field_ui.admin-rtl.css
diff --git a/core/modules/field_ui/field_ui.admin.css b/core/modules/field_ui/css/field_ui.admin.css
similarity index 100%
rename from core/modules/field_ui/field_ui.admin.css
rename to core/modules/field_ui/css/field_ui.admin.css
diff --git a/core/modules/field_ui/field_ui.module b/core/modules/field_ui/field_ui.module
index 51115b8..f3bb151 100644
--- a/core/modules/field_ui/field_ui.module
+++ b/core/modules/field_ui/field_ui.module
@@ -338,7 +338,7 @@ function field_ui_library_info() {
       drupal_get_path('module', 'field_ui') . '/field_ui.js' => array(),
     ),
     'css' => array(
-      drupal_get_path('module', 'field_ui') . '/field_ui.admin.css' => array(),
+      drupal_get_path('module', 'field_ui') . '/css/field_ui.admin.css' => array(),
     ),
     'dependencies' => array(
       array('system', 'jquery'),
diff --git a/core/modules/file/file.admin.css b/core/modules/file/css/file.admin.css
similarity index 100%
rename from core/modules/file/file.admin.css
rename to core/modules/file/css/file.admin.css
diff --git a/core/modules/file/file.module b/core/modules/file/file.module
index 09ad82d..65b15e5 100644
--- a/core/modules/file/file.module
+++ b/core/modules/file/file.module
@@ -1651,7 +1651,7 @@ function file_library_info() {
       drupal_get_path('module', 'file') . '/file.js' => array(),
     ),
     'css' => array(
-      drupal_get_path('module', 'file') . '/file.admin.css'
+      drupal_get_path('module', 'file') . '/css/file.admin.css'
     ),
     'dependencies' => array(
       array('system', 'jquery'),
diff --git a/core/modules/filter/filter.admin-rtl.css b/core/modules/filter/css/filter.admin-rtl.css
similarity index 100%
rename from core/modules/filter/filter.admin-rtl.css
rename to core/modules/filter/css/filter.admin-rtl.css
diff --git a/core/modules/filter/filter.admin.css b/core/modules/filter/css/filter.admin.css
similarity index 89%
rename from core/modules/filter/filter.admin.css
rename to core/modules/filter/css/filter.admin.css
index 8c144cf..ac21686 100644
--- a/core/modules/filter/filter.admin.css
+++ b/core/modules/filter/css/filter.admin.css
@@ -31,7 +31,7 @@
   margin: 0;
 }
 .filter-help a {
-  background: transparent url(../../misc/help.png) right center no-repeat; /* LTR */
+  background: transparent url(../../../misc/help.png) right center no-repeat; /* LTR */
   padding: 0 20px 0 0; /* LTR */
 }
 
diff --git a/core/modules/filter/filter.module b/core/modules/filter/filter.module
index 857a33b..cf0eaef 100644
--- a/core/modules/filter/filter.module
+++ b/core/modules/filter/filter.module
@@ -1439,7 +1439,7 @@ function filter_library_info() {
       drupal_get_path('module', 'filter') . '/filter.admin.js' => array(),
     ),
     'css' => array(
-      drupal_get_path('module', 'filter') . '/filter.admin.css'
+      drupal_get_path('module', 'filter') . '/css/filter.admin.css'
     ),
     'dependencies' => array(
       array('system', 'jquery'),
@@ -1455,7 +1455,7 @@ function filter_library_info() {
       drupal_get_path('module', 'filter') . '/filter.js' => array(),
     ),
     'css' => array(
-      drupal_get_path('module', 'filter') . '/filter.admin.css'
+      drupal_get_path('module', 'filter') . '/css/filter.admin.css'
     ),
     'dependencies' => array(
       array('system', 'jquery'),
diff --git a/core/modules/forum/forum-rtl.css b/core/modules/forum/css/forum.module-rtl.css
similarity index 100%
rename from core/modules/forum/forum-rtl.css
rename to core/modules/forum/css/forum.module-rtl.css
diff --git a/core/modules/forum/forum.css b/core/modules/forum/css/forum.module.css
similarity index 93%
rename from core/modules/forum/forum.css
rename to core/modules/forum/css/forum.module.css
index 5ac5e75..0c4f8fc 100644
--- a/core/modules/forum/forum.css
+++ b/core/modules/forum/css/forum.module.css
@@ -16,7 +16,7 @@
   white-space: nowrap;
 }
 #forum .icon{
-  background-image: url(../../misc/forum-icons.png);
+  background-image: url(../../../misc/forum-icons.png);
   background-repeat: no-repeat;
   float: left; /* LTR */
   height: 24px;
diff --git a/core/modules/forum/forum.pages.inc b/core/modules/forum/forum.pages.inc
index 422d92e..67b7f21 100644
--- a/core/modules/forum/forum.pages.inc
+++ b/core/modules/forum/forum.pages.inc
@@ -73,6 +73,6 @@ function forum_page($forum_term = NULL) {
     '#sortby' => $sort_by,
     '#forums_per_page' => $forum_per_page,
   );
-  $build['#attached']['css'][] = drupal_get_path('module', 'forum') . '/forum.css';
+  $build['#attached']['css'][] = drupal_get_path('module', 'forum') . '/css/forum.module.css';
   return $build;
 }
diff --git a/core/modules/help/help-rtl.css b/core/modules/help/css/help.module-rtl.css
similarity index 100%
rename from core/modules/help/help-rtl.css
rename to core/modules/help/css/help.module-rtl.css
diff --git a/core/modules/help/help.css b/core/modules/help/css/help.module.css
similarity index 100%
rename from core/modules/help/help.css
rename to core/modules/help/css/help.module.css
diff --git a/core/modules/help/lib/Drupal/help/Controller/HelpController.php b/core/modules/help/lib/Drupal/help/Controller/HelpController.php
index 2038b8d..bfe95f6 100644
--- a/core/modules/help/lib/Drupal/help/Controller/HelpController.php
+++ b/core/modules/help/lib/Drupal/help/Controller/HelpController.php
@@ -48,7 +48,7 @@ public static function create(ContainerInterface $container) {
   public function helpMain() {
     $output = array(
       '#attached' => array(
-        'css' => array(drupal_get_path('module', 'help') . '/help.css'),
+        'css' => array(drupal_get_path('module', 'help') . '/css/help.module.css'),
       ),
       '#markup' => '<h2>' . t('Help topics') . '</h2><p>' . t('Help is available on the following items:') . '</p>' . $this->helpLinksAsList(),
     );
diff --git a/core/modules/help/lib/Drupal/help/Tests/HelpTest.php b/core/modules/help/lib/Drupal/help/Tests/HelpTest.php
index 54b955f..5a40fa0 100644
--- a/core/modules/help/lib/Drupal/help/Tests/HelpTest.php
+++ b/core/modules/help/lib/Drupal/help/Tests/HelpTest.php
@@ -67,7 +67,7 @@ public function testHelp() {
     // Check for css on admin/help.
     $this->drupalLogin($this->adminUser);
     $this->drupalGet('admin/help');
-    $this->assertRaw(drupal_get_path('module', 'help') . '/help.css', 'The help.css file is present in the HTML.');
+    $this->assertRaw(drupal_get_path('module', 'help') . '/css/help.module.css', 'The help.module.css file is present in the HTML.');
 
     // Verify that introductory help text exists, goes for 100% module coverage.
     $this->assertRaw(t('For more information, refer to the specific topics listed in the next section or to the <a href="@drupal">online Drupal handbooks</a>.', array('@drupal' => 'http://drupal.org/documentation')), 'Help intro text correctly appears.');
diff --git a/core/modules/image/image.admin.css b/core/modules/image/css/image.admin.css
similarity index 100%
rename from core/modules/image/image.admin.css
rename to core/modules/image/css/image.admin.css
diff --git a/core/modules/image/image.theme-rtl.css b/core/modules/image/css/image.theme-rtl.css
similarity index 100%
rename from core/modules/image/image.theme-rtl.css
rename to core/modules/image/css/image.theme-rtl.css
diff --git a/core/modules/image/image.theme.css b/core/modules/image/css/image.theme.css
similarity index 100%
rename from core/modules/image/image.theme.css
rename to core/modules/image/css/image.theme.css
diff --git a/core/modules/image/image.admin.inc b/core/modules/image/image.admin.inc
index 9ed3aa9..59cc1c6 100644
--- a/core/modules/image/image.admin.inc
+++ b/core/modules/image/image.admin.inc
@@ -15,7 +15,7 @@ function image_style_list() {
   $page['image_style_list'] = array(
     '#markup' => theme('image_style_list', array('styles' => $styles)),
     '#attached' => array(
-      'css' => array(drupal_get_path('module', 'image') . '/image.admin.css' => array()),
+      'css' => array(drupal_get_path('module', 'image') . '/css/image.admin.css' => array()),
     ),
   );
 
@@ -39,7 +39,7 @@ function image_style_form($form, &$form_state, $style) {
 
   $form_state['image_style'] = $style;
   $form['#tree'] = TRUE;
-  $form['#attached']['css'][drupal_get_path('module', 'image') . '/image.admin.css'] = array();
+  $form['#attached']['css'][drupal_get_path('module', 'image') . '/css/image.admin.css'] = array();
 
   // Show the thumbnail preview.
   $form['preview'] = array(
@@ -297,7 +297,7 @@ function image_effect_form($form, &$form_state, $style, $effect) {
   }
   drupal_set_title($title, PASS_THROUGH);
 
-  $form['#attached']['css'][drupal_get_path('module', 'image') . '/image.admin.css'] = array();
+  $form['#attached']['css'][drupal_get_path('module', 'image') . '/css/image.admin.css'] = array();
 
   $form['ieid'] = array(
     '#type' => 'value',
diff --git a/core/modules/image/image.field.inc b/core/modules/image/image.field.inc
index eed43df..705bcf3 100644
--- a/core/modules/image/image.field.inc
+++ b/core/modules/image/image.field.inc
@@ -316,7 +316,7 @@ function image_field_widget_process($element, &$form_state, $form) {
   $item['fids'] = $element['fids']['#value'];
 
   $element['#theme'] = 'image_widget';
-  $element['#attached']['css'][] = drupal_get_path('module', 'image') . '/image.theme.css';
+  $element['#attached']['css'][] = drupal_get_path('module', 'image') . '/css/image.theme.css';
 
   // Add the image preview.
   if (!empty($element['#files']) && $element['#preview_image_style']) {
diff --git a/core/modules/language/language.admin.css b/core/modules/language/css/language.admin.css
similarity index 100%
rename from core/modules/language/language.admin.css
rename to core/modules/language/css/language.admin.css
diff --git a/core/modules/language/language.admin.inc b/core/modules/language/language.admin.inc
index 33eaa6b..68986f6 100644
--- a/core/modules/language/language.admin.inc
+++ b/core/modules/language/language.admin.inc
@@ -805,7 +805,7 @@ function language_content_settings_form(array $form, array $form_state, array $s
   $form = array(
     '#labels' => $labels,
     '#attached' => array(
-      'css' => array($path . '/language.admin.css'),
+      'css' => array($path . '/css/language.admin.css'),
     ),
   );
 
diff --git a/core/modules/layout/layout.admin.css b/core/modules/layout/css/layout.admin.css
similarity index 100%
rename from core/modules/layout/layout.admin.css
rename to core/modules/layout/css/layout.admin.css
diff --git a/core/modules/layout/layout.admin.inc b/core/modules/layout/layout.admin.inc
index 797c4c7..fac4370 100644
--- a/core/modules/layout/layout.admin.inc
+++ b/core/modules/layout/layout.admin.inc
@@ -30,6 +30,6 @@ function layout_page_view($key) {
     '#type' => 'markup',
     '#markup' => $instance->renderLayout(TRUE, $regions),
   );
-  $build['#attached']['css'][] = drupal_get_path('module', 'layout') . '/layout.admin.css';
+  $build['#attached']['css'][] = drupal_get_path('module', 'layout') . '/css/layout.admin.css';
   return $build;
 }
diff --git a/core/modules/locale/locale.admin-rtl.css b/core/modules/locale/css/locale.admin-rtl.css
similarity index 100%
rename from core/modules/locale/locale.admin-rtl.css
rename to core/modules/locale/css/locale.admin-rtl.css
diff --git a/core/modules/locale/locale.admin.css b/core/modules/locale/css/locale.admin.css
similarity index 94%
rename from core/modules/locale/locale.admin.css
rename to core/modules/locale/css/locale.admin.css
index 03ce7bd..d7bd328 100644
--- a/core/modules/locale/locale.admin.css
+++ b/core/modules/locale/css/locale.admin.css
@@ -66,12 +66,12 @@
   vertical-align: top;
 }
 #locale-translation-status-form .expand .inner {
-  background: transparent url(../../misc/menu-collapsed.png) left .6em no-repeat;
+  background: transparent url(../../../misc/menu-collapsed.png) left .6em no-repeat;
   margin-left: -12px;
   padding-left: 12px;
 }
 #locale-translation-status-form .expanded .expand .inner {
-  background: transparent url(../../misc/menu-expanded.png) left .6em no-repeat;
+  background: transparent url(../../../misc/menu-expanded.png) left .6em no-repeat;
 }
 
 #locale-translation-status-form label {
diff --git a/core/modules/locale/locale.pages.inc b/core/modules/locale/locale.pages.inc
index 9ba7b71..5a3a012 100644
--- a/core/modules/locale/locale.pages.inc
+++ b/core/modules/locale/locale.pages.inc
@@ -164,7 +164,7 @@ function locale_translate_filter_form($form, &$form_state) {
   $filter_values = locale_translate_filter_values();
 
   $form['#attached']['css'] = array(
-    drupal_get_path('module', 'locale') . '/locale.admin.css',
+    drupal_get_path('module', 'locale') . '/css/locale.admin.css',
   );
 
   $form['filters'] = array(
@@ -261,7 +261,7 @@ function locale_translate_edit_form($form, &$form_state) {
 
   $path = drupal_get_path('module', 'locale');
   $form['#attached']['css'] = array(
-    $path . '/locale.admin.css',
+    $path . '/css/locale.admin.css',
   );
   $form['#attached']['library'][] = array('locale', 'drupal.locale.admin');
 
@@ -584,7 +584,7 @@ function locale_translation_status_form($form, &$form_state) {
   );
 
   $form['#attached']['library'][] = array('locale', 'drupal.locale.admin');
-  $form['#attached']['css'] = array(drupal_get_path('module', 'locale') . '/locale.admin.css');
+  $form['#attached']['css'] = array(drupal_get_path('module', 'locale') . '/css/locale.admin.css');
 
   $form['actions'] = array('#type' => 'actions');
   if ($languages_update) {
diff --git a/core/modules/menu/menu.admin.css b/core/modules/menu/css/menu.admin.css
similarity index 100%
rename from core/modules/menu/menu.admin.css
rename to core/modules/menu/css/menu.admin.css
diff --git a/core/modules/menu/lib/Drupal/menu/MenuListController.php b/core/modules/menu/lib/Drupal/menu/MenuListController.php
index e557d92..1ed2200 100644
--- a/core/modules/menu/lib/Drupal/menu/MenuListController.php
+++ b/core/modules/menu/lib/Drupal/menu/MenuListController.php
@@ -71,7 +71,7 @@ public function getOperations(EntityInterface $entity) {
    */
   public function render() {
     $build = parent::render();
-    $build['#attached']['css'][] = drupal_get_path('module', 'menu') . '/menu.admin.css';
+    $build['#attached']['css'][] = drupal_get_path('module', 'menu') . '/css/menu.admin.css';
     return $build;
   }
 
diff --git a/core/modules/menu/menu.admin.inc b/core/modules/menu/menu.admin.inc
index 4150b99..f08ddbd 100644
--- a/core/modules/menu/menu.admin.inc
+++ b/core/modules/menu/menu.admin.inc
@@ -71,7 +71,7 @@ function menu_overview_form($form, &$form_state) {
   $form['#theme'] = 'menu_overview_form';
   $form_state += array('menu_overview_form_parents' => array());
 
-  $form['#attached']['css'] = array(drupal_get_path('module', 'menu') . '/menu.admin.css');
+  $form['#attached']['css'] = array(drupal_get_path('module', 'menu') . '/css/menu.admin.css');
 
   $links = array();
   $query = Drupal::entityQuery('menu_link')
diff --git a/core/modules/node/node.admin.css b/core/modules/node/css/node.admin.css
similarity index 100%
rename from core/modules/node/node.admin.css
rename to core/modules/node/css/node.admin.css
diff --git a/core/modules/node/node.edit.admin.css b/core/modules/node/css/node.module.css
similarity index 100%
rename from core/modules/node/node.edit.admin.css
rename to core/modules/node/css/node.module.css
diff --git a/core/modules/node/node.pages.inc b/core/modules/node/node.pages.inc
index f2391be..d8f8d7c 100644
--- a/core/modules/node/node.pages.inc
+++ b/core/modules/node/node.pages.inc
@@ -300,7 +300,7 @@ function node_revision_overview($node) {
     '#rows' => $rows,
     '#header' => $header,
     '#attached' => array (
-      'css' => array(drupal_get_path('module', 'node') . '/node.admin.css'),
+      'css' => array(drupal_get_path('module', 'node') . '/css/node.admin.css'),
     ),
   );
 
diff --git a/core/modules/overlay/overlay-child-rtl.css b/core/modules/overlay/css/overlay-child-rtl.css
similarity index 100%
rename from core/modules/overlay/overlay-child-rtl.css
rename to core/modules/overlay/css/overlay-child-rtl.css
diff --git a/core/modules/overlay/overlay-child.css b/core/modules/overlay/css/overlay-child.css
similarity index 97%
rename from core/modules/overlay/overlay-child.css
rename to core/modules/overlay/css/overlay-child.css
index 48ced7f..4610749 100644
--- a/core/modules/overlay/overlay-child.css
+++ b/core/modules/overlay/css/overlay-child.css
@@ -63,7 +63,7 @@
 }
 #overlay-close,
 #overlay-close:hover {
-  background: #ffffff url(images/close.png) no-repeat;
+  background: #ffffff url(../images/close.png) no-repeat;
   border-radius: 0 12px 12px 0; /* LTR */
   display: block;
   height: 26px;
diff --git a/core/modules/overlay/overlay-parent.css b/core/modules/overlay/css/overlay-parent.css
similarity index 94%
rename from core/modules/overlay/overlay-parent.css
rename to core/modules/overlay/css/overlay-parent.css
index 9459a7a..0447f53 100644
--- a/core/modules/overlay/overlay-parent.css
+++ b/core/modules/overlay/css/overlay-parent.css
@@ -23,7 +23,7 @@ html.overlay-open body {
 
 .overlay-modal-background {
   /* Using a transparent png renders faster than using opacity */
-  background: transparent url(images/background.png) repeat;
+  background: transparent url(../images/background.png) repeat;
 }
 
 .overlay-element {
diff --git a/core/modules/overlay/overlay.module b/core/modules/overlay/overlay.module
index d34f175..ce4092b 100644
--- a/core/modules/overlay/overlay.module
+++ b/core/modules/overlay/overlay.module
@@ -140,7 +140,7 @@ function overlay_library_info() {
       $module_path . '/overlay-parent.js' => array(),
     ),
     'css' => array(
-      $module_path . '/overlay-parent.css' => array(),
+      $module_path . '/css/overlay-parent.css' => array(),
     ),
     'dependencies' => array(
       array('system', 'jquery'),
@@ -162,7 +162,7 @@ function overlay_library_info() {
       $module_path . '/overlay-child.js' => array(),
     ),
     'css' => array(
-      $module_path . '/overlay-child.css' => array(),
+      $module_path . '/css/overlay-child.css' => array(),
     ),
     'dependencies' => array(
       array('system', 'jquery'),
diff --git a/core/modules/shortcut/shortcut.base-rtl.css b/core/modules/shortcut/css/shortcut.module-rtl.css
similarity index 100%
rename from core/modules/shortcut/shortcut.base-rtl.css
rename to core/modules/shortcut/css/shortcut.module-rtl.css
diff --git a/core/modules/shortcut/shortcut.base.css b/core/modules/shortcut/css/shortcut.module.css
similarity index 100%
rename from core/modules/shortcut/shortcut.base.css
rename to core/modules/shortcut/css/shortcut.module.css
diff --git a/core/modules/shortcut/shortcut.theme-rtl.css b/core/modules/shortcut/css/shortcut.theme-rtl.css
similarity index 100%
rename from core/modules/shortcut/shortcut.theme-rtl.css
rename to core/modules/shortcut/css/shortcut.theme-rtl.css
diff --git a/core/modules/shortcut/shortcut.theme.css b/core/modules/shortcut/css/shortcut.theme.css
similarity index 83%
rename from core/modules/shortcut/shortcut.theme.css
rename to core/modules/shortcut/css/shortcut.theme.css
index 04f7c9f..a9525a7 100644
--- a/core/modules/shortcut/shortcut.theme.css
+++ b/core/modules/shortcut/css/shortcut.theme.css
@@ -7,11 +7,11 @@
  * Toolbar.
  */
 .icon-shortcut:before {
-  background-image: url("images/shortcut.png");
+  background-image: url(../images/shortcut.png);
 }
 .icon-shortcut:active:before,
 .icon-shortcut.active:before {
-  background-image: url("images/shortcut-active.png");
+  background-image: url(../images/shortcut-active.png);
 }
 .toolbar .tray.horizontal.shortcuts .menu {
   float: left;
@@ -31,7 +31,7 @@
  * Add/remove links.
  */
 .add-or-remove-shortcuts .icon {
-  background: transparent url(shortcut.png) no-repeat;
+  background: transparent url(../images/shortcut-add.png) no-repeat;
   height: 12px;
   margin-left: 8px; /* LTR */
   overflow: hidden;
diff --git a/core/modules/shortcut/shortcut.png b/core/modules/shortcut/images/shortcut-add.png
similarity index 100%
rename from core/modules/shortcut/shortcut.png
rename to core/modules/shortcut/images/shortcut-add.png
diff --git a/core/modules/shortcut/shortcut.module b/core/modules/shortcut/shortcut.module
index db284c5..8de14a0 100644
--- a/core/modules/shortcut/shortcut.module
+++ b/core/modules/shortcut/shortcut.module
@@ -524,8 +524,8 @@ function shortcut_preprocess_page(&$variables) {
       $variables['title_suffix']['add_or_remove_shortcut'] = array(
         '#attached' => array(
           'css' => array(
-            drupal_get_path('module', 'shortcut') . '/shortcut.base.css',
-            drupal_get_path('module', 'shortcut') . '/shortcut.theme.css',
+            drupal_get_path('module', 'shortcut') . '/css/shortcut.module.css',
+            drupal_get_path('module', 'shortcut') . '/css/shortcut.theme.css',
           ),
         ),
         '#prefix' => '<div class="add-or-remove-shortcuts ' . $link_mode . '-shortcut">',
@@ -577,8 +577,8 @@ function shortcut_toolbar() {
       '#weight' => -10,
       '#attached' => array(
         'css' => array(
-          drupal_get_path('module', 'shortcut') . '/shortcut.base.css',
-          drupal_get_path('module', 'shortcut') . '/shortcut.theme.css',
+          drupal_get_path('module', 'shortcut') . '/css/shortcut.module.css',
+          drupal_get_path('module', 'shortcut') . '/css/shortcut.theme.css',
         ),
       ),
     );
diff --git a/core/modules/simpletest/simpletest.css b/core/modules/simpletest/css/simpletest.module.css
similarity index 100%
rename from core/modules/simpletest/simpletest.css
rename to core/modules/simpletest/css/simpletest.module.css
diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 06e56e7..73e82ac 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -154,7 +154,7 @@ function simpletest_run_tests($test_list, $reporter = 'drupal') {
     ),
     'finished' => '_simpletest_batch_finished',
     'progress_message' => '',
-    'css' => array(drupal_get_path('module', 'simpletest') . '/simpletest.css'),
+    'css' => array(drupal_get_path('module', 'simpletest') . '/css/simpletest.module.css'),
     'init_message' => t('Processing test @num of @max - %test.', array('%test' => $info['name'], '@num' => '1', '@max' => count($test_list))),
   );
   batch_set($batch);
@@ -692,7 +692,7 @@ function simpletest_library_info() {
       drupal_get_path('module', 'simpletest') . '/simpletest.js' => array(),
     ),
     'css' => array(
-      drupal_get_path('module', 'simpletest') . '/simpletest.css' => array(),
+      drupal_get_path('module', 'simpletest') . '/css/simpletest.module.css' => array(),
     ),
     'dependencies' => array(
       array('system', 'jquery'),
diff --git a/core/modules/system/system.admin-rtl.css b/core/modules/system/css/system.admin-rtl.css
similarity index 100%
rename from core/modules/system/system.admin-rtl.css
rename to core/modules/system/css/system.admin-rtl.css
diff --git a/core/modules/system/system.admin.css b/core/modules/system/css/system.admin.css
similarity index 92%
rename from core/modules/system/system.admin.css
rename to core/modules/system/css/system.admin.css
index a7f2cbd..51c2870 100644
--- a/core/modules/system/system.admin.css
+++ b/core/modules/system/css/system.admin.css
@@ -74,12 +74,12 @@ small .admin-link:after {
   vertical-align: top;
 }
 #system-modules .expand .inner {
-  background: transparent url(../../misc/menu-collapsed.png) left .6em no-repeat;
+  background: transparent url(../../../misc/menu-collapsed.png) left .6em no-repeat;
   margin-left: -12px;
   padding-left: 12px;
 }
 #system-modules .expanded .expand .inner {
-  background: transparent url(../../misc/menu-expanded.png) left .6em no-repeat;
+  background: transparent url(../../../misc/menu-expanded.png) left .6em no-repeat;
 }
 #system-modules label {
   color: #1d1d1d;
@@ -150,13 +150,13 @@ a.module-link {
   float: left; /* LTR */
 }
 a.module-link-help {
-  background: url(../../misc/help.png) 0 50% no-repeat; /* LTR */
+  background: url(../../../misc/help.png) 0 50% no-repeat; /* LTR */
 }
 a.module-link-permissions {
-  background: url(../../misc/permissions.png) 0 50% no-repeat; /* LTR */
+  background: url(../../../misc/permissions.png) 0 50% no-repeat; /* LTR */
 }
 a.module-link-configure {
-  background: url(../../misc/configure.png) 0 50% no-repeat; /* LTR */
+  background: url(../../../misc/configure.png) 0 50% no-repeat; /* LTR */
 }
 .module-help {
   margin-left: 1em; /* LTR */
@@ -183,10 +183,10 @@ table.system-status-report td.status-icon div {
   width: 16px;
 }
 table.system-status-report tr.error td.status-icon div {
-  background-image: url(../../misc/message-16-error.png);
+  background-image: url(../../../misc/message-16-error.png);
 }
 table.system-status-report tr.warning td.status-icon div {
-  background-image: url(../../misc/message-16-warning.png);
+  background-image: url(../../../misc/message-16-warning.png);
 }
 table.system-status-report .status-title {
   width: 25%;
diff --git a/core/modules/system/system.diff.css b/core/modules/system/css/system.diff.css
similarity index 100%
rename from core/modules/system/system.diff.css
rename to core/modules/system/css/system.diff.css
diff --git a/core/modules/system/system.maintenance.css b/core/modules/system/css/system.maintenance.css
similarity index 100%
rename from core/modules/system/system.maintenance.css
rename to core/modules/system/css/system.maintenance.css
diff --git a/core/modules/system/system.base-rtl.css b/core/modules/system/css/system.module-rtl.css
similarity index 100%
rename from core/modules/system/system.base-rtl.css
rename to core/modules/system/css/system.module-rtl.css
diff --git a/core/modules/system/system.base.css b/core/modules/system/css/system.module.css
similarity index 93%
rename from core/modules/system/system.base.css
rename to core/modules/system/css/system.module.css
index 08ff66c..9a1c950 100644
--- a/core/modules/system/system.base.css
+++ b/core/modules/system/css/system.module.css
@@ -30,7 +30,7 @@
 
 /* Animated throbber */
 .js input.form-autocomplete {
-  background-image: url(../../misc/throbber.gif);
+  background-image: url(../../../misc/throbber.gif);
   background-position: 100% 2px; /* LTR */
   background-repeat: no-repeat;
 }
@@ -95,7 +95,7 @@ a.tabledrag-handle:hover {
   text-decoration: none;
 }
 a.tabledrag-handle .handle {
-  background: url(../../misc/draggable.png) no-repeat 6px 9px;
+  background: url(../../../misc/draggable.png) no-repeat 6px 9px;
   height: 13px;
   margin: -0.4em 0.5em; /* LTR */
   padding: 0.42em 0.5em; /* LTR */
@@ -131,13 +131,13 @@ div.indentation {
   width: 20px;
 }
 div.tree-child {
-  background: url(../../misc/tree.png) no-repeat 11px center; /* LTR */
+  background: url(../../../misc/tree.png) no-repeat 11px center; /* LTR */
 }
 div.tree-child-last {
-  background: url(../../misc/tree-bottom.png) no-repeat 11px center; /* LTR */
+  background: url(../../../misc/tree-bottom.png) no-repeat 11px center; /* LTR */
 }
 div.tree-child-horizontal {
-  background: url(../../misc/tree.png) no-repeat -11px center;
+  background: url(../../../misc/tree.png) no-repeat -11px center;
 }
 .tabledrag-toggle-weight-wrapper {
   text-align: right; /* LTR */
@@ -178,7 +178,7 @@ table.sticky-header {
   padding: 1px 5px 2px 5px;
 }
 .ajax-progress-throbber .throbber {
-  background: transparent url(../../misc/throbber.gif) no-repeat 0px -18px;
+  background: transparent url(../../../misc/throbber.gif) no-repeat 0px -18px;
   display: inline;
   padding: 1px 5px 2px;
 }
diff --git a/core/modules/system/system.plugin.ui.css b/core/modules/system/css/system.plugin.ui.css
similarity index 100%
rename from core/modules/system/system.plugin.ui.css
rename to core/modules/system/css/system.plugin.ui.css
diff --git a/core/modules/system/system.theme-rtl.css b/core/modules/system/css/system.theme-rtl.css
similarity index 96%
rename from core/modules/system/system.theme-rtl.css
rename to core/modules/system/css/system.theme-rtl.css
index eb50ab3..789d908 100644
--- a/core/modules/system/system.theme-rtl.css
+++ b/core/modules/system/css/system.theme-rtl.css
@@ -67,7 +67,7 @@ ul.menu {
   text-align: right;
 }
 .menu .collapsed {
-  list-style-image: url(../../misc/menu-collapsed-rtl.png);
+  list-style-image: url(../../../misc/menu-collapsed-rtl.png);
 }
 
 /**
diff --git a/core/modules/system/system.theme.css b/core/modules/system/css/system.theme.css
similarity index 93%
rename from core/modules/system/system.theme.css
rename to core/modules/system/css/system.theme.css
index a3c770a..c67d1a1 100644
--- a/core/modules/system/system.theme.css
+++ b/core/modules/system/css/system.theme.css
@@ -172,7 +172,7 @@ abbr.form-required, abbr.tabledrag-changed, abbr.ajax-changed {
   text-align: right; /* LTR */
 }
 .more-help-link a {
-  background: url(../../misc/help.png) 0 50% no-repeat; /* LTR */
+  background: url(../../../misc/help.png) 0 50% no-repeat; /* LTR */
   padding: 1px 0 1px 20px; /* LTR */
 }
 
@@ -245,7 +245,7 @@ summary {
   padding-right: 0.5em;
 }
 .collapse-processed > summary:before {
-  background: url(../../misc/menu-expanded.png) 0px 100% no-repeat; /* LTR */
+  background: url(../../../misc/menu-expanded.png) 0px 100% no-repeat; /* LTR */
   content: "";
   float: left;
   height: 1em;
@@ -303,7 +303,7 @@ th.checkbox {
   border-radius: 3px;
 }
 .progress .filled {
-  background: #0072b9 url(../../misc/progress.gif);
+  background: #0072b9 url(../../../misc/progress.gif);
 }
 
 /**
@@ -316,11 +316,11 @@ ul.menu {
   text-align: left; /* LTR */
 }
 .menu .expanded {
-  list-style-image: url(../../misc/menu-expanded.png);
+  list-style-image: url(../../../misc/menu-expanded.png);
   list-style-type: circle;
 }
 .menu .collapsed {
-  list-style-image: url(../../misc/menu-collapsed.png); /* LTR */
+  list-style-image: url(../../../misc/menu-collapsed.png); /* LTR */
   list-style-type: disc;
 }
 ul.menu li {
@@ -452,7 +452,7 @@ ul.tabs {
 /* @TODO Separate tables and messages styling */
 .messages--status {
   border-color: #c9e1bd #c9e1bd #c9e1bd #77b259;  /* LTR */
-  background-image: url(../../misc/message-16-ok.png);
+  background-image: url(../../../misc/message-16-ok.png);
   background-position: 12px 19px;  /* LTR */
 }
 .messages--status,
@@ -465,7 +465,7 @@ table tr.ok {
 }
 
 .messages--warning {
-  background-image: url(../../misc/message-16-warning.png);
+  background-image: url(../../../misc/message-16-warning.png);
   border-color: #f4daa6 #f4daa6 #f4daa6 #e09600;  /* LTR */
 }
 .messages--warning,
@@ -478,7 +478,7 @@ table tr.warning {
 }
 
 .messages--error {
-  background-image: url(../../misc/message-16-error.png);
+  background-image: url(../../../misc/message-16-error.png);
   border-color: #f9c9bf #f9c9bf #f9c9bf #e62600;  /* LTR */
 }
 .messages--error,
diff --git a/core/modules/system/lib/Drupal/system/Tests/Ajax/FrameworkTest.php b/core/modules/system/lib/Drupal/system/Tests/Ajax/FrameworkTest.php
index e7fd51a..e30b55c 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Ajax/FrameworkTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Ajax/FrameworkTest.php
@@ -47,8 +47,8 @@ function testOrder() {
     // Expected commands, in a very specific order.
     $expected_commands[0] = new SettingsCommand(array('ajax' => 'test'), TRUE);
     drupal_static_reset('drupal_add_css');
-    drupal_add_css($path . '/system.admin.css');
-    drupal_add_css($path . '/system.maintenance.css');
+    drupal_add_css($path . '/css/system.admin.css');
+    drupal_add_css($path . '/css/system.maintenance.css');
     $expected_commands[1] = new AddCssCommand(drupal_get_css(drupal_add_css(), TRUE));
     drupal_static_reset('drupal_add_js');
     drupal_add_js($path . '/system.js');
@@ -99,7 +99,7 @@ function testLazyLoad() {
     $expected = array(
       'setting_name' => 'ajax_forms_test_lazy_load_form_submit',
       'setting_value' => 'executed',
-      'css' => drupal_get_path('module', 'system') . '/system.admin.css',
+      'css' => drupal_get_path('module', 'system') . '/css/system.admin.css',
       'js' => drupal_get_path('module', 'system') . '/system.js',
     );
     // CSS files are stored by basename, see drupal_add_css().
@@ -188,7 +188,7 @@ function testLazyLoad() {
    * Tests that overridden CSS files are not added during lazy load.
    */
   function testLazyLoadOverriddenCSS() {
-    // The test theme overrides system.base.css without an implementation,
+    // The test theme overrides system.module.css without an implementation,
     // thereby removing it.
     theme_enable(array('test_theme'));
     config('system.theme')
@@ -204,6 +204,6 @@ function testLazyLoadOverriddenCSS() {
     // information about the file; we only really care about whether it appears
     // in a LINK or STYLE tag, for which Drupal always adds a query string for
     // cache control.
-    $this->assertNoText('system.base.css?', 'Ajax lazy loading does not add overridden CSS files.');
+    $this->assertNoText('system.module.css?', 'Ajax lazy loading does not add overridden CSS files.');
   }
 }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Common/CascadingStylesheetsTest.php b/core/modules/system/lib/Drupal/system/Tests/Common/CascadingStylesheetsTest.php
index bb97d52..c546e92 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Common/CascadingStylesheetsTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Common/CascadingStylesheetsTest.php
@@ -47,9 +47,9 @@ function testDefault() {
    * Tests adding a file stylesheet.
    */
   function testAddFile() {
-    $path = drupal_get_path('module', 'simpletest') . '/simpletest.css';
+    $path = drupal_get_path('module', 'simpletest') . '/css/simpletest.module.css';
     $css = drupal_add_css($path);
-    $this->assertEqual($css['simpletest.css']['data'], $path);
+    $this->assertEqual($css['simpletest.module.css']['data'], $path);
   }
 
   /**
@@ -73,7 +73,7 @@ function testReset() {
    * Tests rendering the stylesheets.
    */
   function testRenderFile() {
-    $css = drupal_get_path('module', 'simpletest') . '/simpletest.css';
+    $css = drupal_get_path('module', 'simpletest') . '/css/simpletest.module.css';
     drupal_add_css($css);
     $styles = drupal_get_css();
     $this->assertTrue(strpos($styles, $css) > 0, 'Rendered CSS includes the added stylesheet.');
@@ -154,19 +154,18 @@ function testRenderInlineFullPage() {
    */
   function testRenderOrder() {
     // A module CSS file.
-    drupal_add_css(drupal_get_path('module', 'simpletest') . '/simpletest.css');
+    drupal_add_css(drupal_get_path('module', 'simpletest') . '/css/simpletest.module.css');
     // A few system CSS files, ordered in a strange way.
     $system_path = drupal_get_path('module', 'system');
-    drupal_add_css($system_path . '/system.base.css', array('group' => CSS_AGGREGATE_SYSTEM, 'weight' => -10));
-    drupal_add_css($system_path . '/system.theme.css', array('group' => CSS_AGGREGATE_SYSTEM));
+    drupal_add_css($system_path . '/css/system.module.css', array('group' => CSS_AGGREGATE_SYSTEM, 'weight' => -10));
+    drupal_add_css($system_path . '/css/system.theme.css', array('group' => CSS_AGGREGATE_SYSTEM));
 
     $expected = array(
-      $system_path . '/system.base.css',
-      $system_path . '/system.theme.css',
-      drupal_get_path('module', 'simpletest') . '/simpletest.css',
+      $system_path . '/css/system.module.css',
+      $system_path . '/css/system.theme.css',
+      drupal_get_path('module', 'simpletest') . '/css/simpletest.module.css',
     );
 
-
     $styles = drupal_get_css();
     // Stylesheet URL may be the href of a LINK tag or in an @import statement
     // of a STYLE tag.
@@ -186,36 +185,36 @@ function testRenderOrder() {
   function testRenderOverride() {
     $system = drupal_get_path('module', 'system');
 
-    drupal_add_css($system . '/system.base.css');
-    drupal_add_css($system . '/tests/system.base.css');
+    drupal_add_css($system . '/css/system.module.css');
+    drupal_add_css($system . '/tests/css/system.module.css');
 
     // The dummy stylesheet should be the only one included.
     $styles = drupal_get_css();
-    $this->assert(strpos($styles, $system . '/tests/system.base.css') !== FALSE, 'The overriding CSS file is output.');
-    $this->assert(strpos($styles, $system . '/system.base.css') === FALSE, 'The overridden CSS file is not output.');
+    $this->assert(strpos($styles, $system . '/tests/css/system.module.css') !== FALSE, 'The overriding CSS file is output.');
+    $this->assert(strpos($styles, $system . '/css/system.module.css') === FALSE, 'The overridden CSS file is not output.');
 
-    drupal_add_css($system . '/tests/system.base.css');
-    drupal_add_css($system . '/system.base.css');
+    drupal_add_css($system . '/tests/css/system.module.css');
+    drupal_add_css($system . '/css/system.module.css');
 
     // The standard stylesheet should be the only one included.
     $styles = drupal_get_css();
-    $this->assert(strpos($styles, $system . '/system.base.css') !== FALSE, 'The overriding CSS file is output.');
-    $this->assert(strpos($styles, $system . '/tests/system.base.css') === FALSE, 'The overridden CSS file is not output.');
+    $this->assert(strpos($styles, $system . '/css/system.module.css') !== FALSE, 'The overriding CSS file is output.');
+    $this->assert(strpos($styles, $system . '/tests/css/system.module.css') === FALSE, 'The overridden CSS file is not output.');
   }
 
   /**
    * Tests Locale module's CSS Alter to include RTL overrides.
    */
   function testAlter() {
-    // Switch the language to a right to left language and add system.base.css.
+    // Switch the language to a right to left language and add system.module.css.
     $language_interface = language(Language::TYPE_INTERFACE);
     $language_interface->direction = Language::DIRECTION_RTL;
     $path = drupal_get_path('module', 'system');
-    drupal_add_css($path . '/system.base.css');
+    drupal_add_css($path . '/css/system.module.css');
 
-    // Check to see if system.base-rtl.css was also added.
+    // Check to see if system.module-rtl.css was also added.
     $styles = drupal_get_css();
-    $this->assert(strpos($styles, $path . '/system.base-rtl.css') !== FALSE, 'CSS is alterable as right to left overrides are added.');
+    $this->assert(strpos($styles, $path . '/css/system.module-rtl.css') !== FALSE, 'CSS is alterable as right to left overrides are added.');
 
     // Change the language back to left to right.
     $language_interface->direction = Language::DIRECTION_LTR;
@@ -227,7 +226,7 @@ function testAlter() {
   function testAddCssFileWithQueryString() {
     $this->drupalGet('common-test/query-string');
     $query_string = variable_get('css_js_query_string', '0');
-    $this->assertRaw(drupal_get_path('module', 'node') . '/node.admin.css?' . $query_string, 'Query string was appended correctly to css.');
+    $this->assertRaw(drupal_get_path('module', 'node') . '/css/node.admin.css?' . $query_string, 'Query string was appended correctly to css.');
     $this->assertRaw(drupal_get_path('module', 'node') . '/node-fake.css?arg1=value1&amp;arg2=value2', 'Query string not escaped on a URI.');
   }
 }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
index a773922..5054555 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -115,7 +115,7 @@ function testCSSOverride() {
     $config->set('css.preprocess', 0);
     $config->save();
     $this->drupalGet('theme-test/suggestion');
-    $this->assertNoText('system.base.css', 'The theme\'s .info.yml file is able to override a module CSS file from being added to the page.');
+    $this->assertNoText('system.module.css', 'The theme\'s .info.yml file is able to override a module CSS file from being added to the page.');
 
     // Also test with aggregation enabled, simply ensuring no PHP errors are
     // triggered during drupal_build_css_cache() when a source file doesn't
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index db6ecc1..b6bf820 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -1464,7 +1464,7 @@ function system_library_info() {
       'core/misc/dropbutton/dropbutton.js' => array(),
     ),
     'css' => array(
-      'core/misc/dropbutton/dropbutton.base.css' => array(),
+      'core/misc/dropbutton/dropbutton.css' => array(),
       'core/misc/dropbutton/dropbutton.theme.css' => array(),
     ),
     'dependencies' => array(
@@ -2498,11 +2498,11 @@ function system_filetransfer_info() {
 function system_page_build(&$page) {
   $path = drupal_get_path('module', 'system');
   // Use the CSS_AGGREGATE_SYSTEM group to load these early.
-  $page['#attached']['css'][$path . '/system.base.css'] = array('group' => CSS_AGGREGATE_SYSTEM, 'every_page' => TRUE);
+  $page['#attached']['css'][$path . '/css/system.module.css'] = array('group' => CSS_AGGREGATE_SYSTEM, 'every_page' => TRUE);
   if (path_is_admin(current_path())) {
-    $page['#attached']['css'][$path . '/system.admin.css'] = array('group' => CSS_AGGREGATE_SYSTEM);
+    $page['#attached']['css'][$path . '/css/system.admin.css'] = array('group' => CSS_AGGREGATE_SYSTEM);
   }
-  $page['#attached']['css'][$path . '/system.theme.css'] = array('group' => CSS_AGGREGATE_SYSTEM, 'every_page' => TRUE);
+  $page['#attached']['css'][$path . '/css/system.theme.css'] = array('group' => CSS_AGGREGATE_SYSTEM, 'every_page' => TRUE);
 }
 
 /**
@@ -2633,7 +2633,7 @@ function system_preprocess_block(&$variables) {
  *   - form: The plugin form elements.
 */
 function template_preprocess_system_plugin_ui_form(&$variables) {
-  drupal_add_css(drupal_get_path('module', 'system') . '/system.plugin.ui.css');
+  drupal_add_css(drupal_get_path('module', 'system') . '/css/system.plugin.ui.css');
 }
 
 /**
diff --git a/core/modules/system/tests/system.base.css b/core/modules/system/tests/css/system.module.css
similarity index 100%
rename from core/modules/system/tests/system.base.css
rename to core/modules/system/tests/css/system.module.css
diff --git a/core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module b/core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module
index 90b57be..1682c01 100644
--- a/core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module
+++ b/core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module
@@ -540,7 +540,7 @@ function ajax_forms_test_lazy_load_form($form, &$form_state) {
 function ajax_forms_test_lazy_load_form_submit($form, &$form_state) {
   if ($form_state['values']['add_files']) {
     drupal_add_js(array('ajax_forms_test_lazy_load_form_submit' => 'executed'), 'setting');
-    drupal_add_css(drupal_get_path('module', 'system') . '/system.admin.css');
+    drupal_add_css(drupal_get_path('module', 'system') . '/css/system.admin.css');
     drupal_add_js(drupal_get_path('module', 'system') . '/system.js');
   }
   $form_state['rebuild'] = TRUE;
diff --git a/core/modules/system/tests/modules/ajax_test/ajax_test.module b/core/modules/system/tests/modules/ajax_test/ajax_test.module
index e7a252c..657af2d 100644
--- a/core/modules/system/tests/modules/ajax_test/ajax_test.module
+++ b/core/modules/system/tests/modules/ajax_test/ajax_test.module
@@ -81,8 +81,8 @@ function ajax_test_order() {
   // HTML insertion command.
   $response->addCommand(new HtmlCommand('body', 'Hello, world!'));
   // Add two CSS files (order should remain the same).
-  drupal_add_css($path . '/system.admin.css');
-  drupal_add_css($path . '/system.maintenance.css');
+  drupal_add_css($path . '/css/system.admin.css');
+  drupal_add_css($path . '/css/system.maintenance.css');
   // Add two JavaScript files (first to the footer, should appear last).
   drupal_add_js($path . '/system.modules.js', array('scope' => 'footer'));
   drupal_add_js($path . '/system.js');
diff --git a/core/modules/system/tests/modules/common_test/common_test.module b/core/modules/system/tests/modules/common_test/common_test.module
index ef6f1a1..febfd3a 100644
--- a/core/modules/system/tests/modules/common_test/common_test.module
+++ b/core/modules/system/tests/modules/common_test/common_test.module
@@ -316,7 +316,7 @@ function common_test_library_info() {
 function common_test_js_and_css_querystring() {
    drupal_add_library('system', 'drupalSettings');
    drupal_add_js(drupal_get_path('module', 'node') . '/node.js');
-   drupal_add_css(drupal_get_path('module', 'node') . '/node.admin.css');
+   drupal_add_css(drupal_get_path('module', 'node') . '/css/node.admin.css');
    // A relative URI may have a query string.
    drupal_add_css('/' . drupal_get_path('module', 'node') . '/node-fake.css?arg1=value1&arg2=value2');
    return '';
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.info.yml b/core/modules/system/tests/themes/test_theme/test_theme.info.yml
index c3cc5a8..1503ee0 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.info.yml
+++ b/core/modules/system/tests/themes/test_theme/test_theme.info.yml
@@ -14,6 +14,6 @@ description: 'Theme for testing the theme system'
 core: 8.x
 hidden: true
 stylesheets-remove:
-  - system.base.css
+  - system.module.css
 settings:
   theme_test_setting: 'default value'
diff --git a/core/modules/taxonomy/taxonomy.css b/core/modules/taxonomy/css/taxonomy.module.css
similarity index 100%
rename from core/modules/taxonomy/taxonomy.css
rename to core/modules/taxonomy/css/taxonomy.module.css
diff --git a/core/modules/taxonomy/lib/Drupal/taxonomy/TermRenderController.php b/core/modules/taxonomy/lib/Drupal/taxonomy/TermRenderController.php
index 4b6c60f..e7f8882 100644
--- a/core/modules/taxonomy/lib/Drupal/taxonomy/TermRenderController.php
+++ b/core/modules/taxonomy/lib/Drupal/taxonomy/TermRenderController.php
@@ -53,7 +53,7 @@ protected function getBuildDefaults(EntityInterface $entity, $view_mode, $langco
    */
   protected function alterBuild(array &$build, EntityInterface $entity, EntityDisplay $display, $view_mode, $langcode = NULL) {
     parent::alterBuild($build, $entity, $display, $view_mode, $langcode);
-    $build['#attached']['css'][] = drupal_get_path('module', 'taxonomy') . '/taxonomy.css';
+    $build['#attached']['css'][] = drupal_get_path('module', 'taxonomy') . '/css/taxonomy.module.css';
     $build['#contextual_links']['taxonomy'] = array('taxonomy/term', array($entity->id()));
   }
 
diff --git a/core/modules/taxonomy/taxonomy.module b/core/modules/taxonomy/taxonomy.module
index f1c2e01..8865e17 100644
--- a/core/modules/taxonomy/taxonomy.module
+++ b/core/modules/taxonomy/taxonomy.module
@@ -1297,7 +1297,7 @@ function taxonomy_library_info() {
       drupal_get_path('module', 'taxonomy') . '/taxonomy.js' => array(),
     ),
     'css' => array(
-      drupal_get_path('module', 'taxonomy') . '/taxonomy.css' => array(),
+      drupal_get_path('module', 'taxonomy') . '/css/taxonomy.module.css' => array(),
     ),
     'dependencies' => array(
       array('system', 'jquery'),
diff --git a/core/modules/toolbar/css/toolbar.base-rtl.css b/core/modules/toolbar/css/toolbar.module-rtl.css
similarity index 97%
rename from core/modules/toolbar/css/toolbar.base-rtl.css
rename to core/modules/toolbar/css/toolbar.module-rtl.css
index cbeecd9..ae80d8a 100644
--- a/core/modules/toolbar/css/toolbar.base-rtl.css
+++ b/core/modules/toolbar/css/toolbar.module-rtl.css
@@ -1,5 +1,5 @@
 /**
- * @file toolbar.base-rtl.css
+ * @file toolbar.module-rtl.css
  */
 html.js .toolbar {
   left: auto;
diff --git a/core/modules/toolbar/css/toolbar.base.css b/core/modules/toolbar/css/toolbar.module.css
similarity index 99%
rename from core/modules/toolbar/css/toolbar.base.css
rename to core/modules/toolbar/css/toolbar.module.css
index 64e6468..9406f86 100644
--- a/core/modules/toolbar/css/toolbar.base.css
+++ b/core/modules/toolbar/css/toolbar.module.css
@@ -1,5 +1,5 @@
 /**
- * @file toolbar.base.css
+ * @file toolbar.module.css
  *
  *
  * Aggressive resets so we can achieve a consistent look in hostile CSS
diff --git a/core/modules/toolbar/toolbar.module b/core/modules/toolbar/toolbar.module
index c9f0c22..1996c97 100644
--- a/core/modules/toolbar/toolbar.module
+++ b/core/modules/toolbar/toolbar.module
@@ -586,7 +586,7 @@ function toolbar_library_info() {
       drupal_get_path('module', 'toolbar') . '/js/toolbar.js' => array(),
     ),
     'css' => array(
-      drupal_get_path('module', 'toolbar') . '/css/toolbar.base.css',
+      drupal_get_path('module', 'toolbar') . '/css/toolbar.module.css',
       drupal_get_path('module', 'toolbar') . '/css/toolbar.theme.css',
       drupal_get_path('module', 'toolbar') . '/css/toolbar.icons.css',
     ),
diff --git a/core/modules/tour/css/tour-rtl.css b/core/modules/tour/css/tour.module-rtl.css
similarity index 100%
rename from core/modules/tour/css/tour-rtl.css
rename to core/modules/tour/css/tour.module-rtl.css
diff --git a/core/modules/tour/css/tour.css b/core/modules/tour/css/tour.module.css
similarity index 100%
rename from core/modules/tour/css/tour.css
rename to core/modules/tour/css/tour.module.css
diff --git a/core/modules/tour/tour.module b/core/modules/tour/tour.module
index fe9bc1a..8d96cfd 100644
--- a/core/modules/tour/tour.module
+++ b/core/modules/tour/tour.module
@@ -45,7 +45,7 @@ function tour_library_info() {
     'title' => 'Tour',
     'version' => VERSION,
     'css' => array(
-      $path . '/css/tour.css' => array('media' => 'screen'),
+      $path . '/css/tour.module.css' => array('media' => 'screen'),
     )
   );
 
diff --git a/core/modules/translation_entity/translation_entity.admin.css b/core/modules/translation_entity/css/translation_entity.admin.css
similarity index 100%
rename from core/modules/translation_entity/translation_entity.admin.css
rename to core/modules/translation_entity/css/translation_entity.admin.css
diff --git a/core/modules/translation_entity/translation_entity.module b/core/modules/translation_entity/translation_entity.module
index f51c2cb..c5cfc6c 100644
--- a/core/modules/translation_entity/translation_entity.module
+++ b/core/modules/translation_entity/translation_entity.module
@@ -362,7 +362,7 @@ function translation_entity_library_info() {
       $path . '/translation_entity.admin.js' => array(),
     ),
     'css' => array(
-      $path . '/translation_entity.admin.css' => array(),
+      $path . '/css/translation_entity.admin.css' => array(),
     ),
     'dependencies' => array(
       array('system', 'jquery'),
diff --git a/core/modules/update/update-rtl.css b/core/modules/update/css/update.admin-rtl.css
similarity index 100%
rename from core/modules/update/update-rtl.css
rename to core/modules/update/css/update.admin-rtl.css
diff --git a/core/modules/update/update.css b/core/modules/update/css/update.admin.css
similarity index 100%
rename from core/modules/update/update.css
rename to core/modules/update/css/update.admin.css
diff --git a/core/modules/update/update.manager.inc b/core/modules/update/update.manager.inc
index fc07b00..c7502a2 100644
--- a/core/modules/update/update.manager.inc
+++ b/core/modules/update/update.manager.inc
@@ -77,7 +77,7 @@ function update_manager_update_form($form, $form_state = array(), $context) {
     return $form;
   }
 
-  $form['#attached']['css'][] = drupal_get_path('module', 'update') . '/update.css';
+  $form['#attached']['css'][] = drupal_get_path('module', 'update') . '/css/update.admin.css';
 
   // This will be a nested array. The first key is the kind of project, which
   // can be either 'enabled', 'disabled', 'manual' (projects which require
diff --git a/core/modules/update/update.report.inc b/core/modules/update/update.report.inc
index ea778d7..d21a5cb 100644
--- a/core/modules/update/update.report.inc
+++ b/core/modules/update/update.report.inc
@@ -231,7 +231,7 @@ function theme_update_report($variables) {
       $output .= theme('table', array('header' => $header, 'rows' => $rows[$type_name], 'attributes' => array('class' => array('update'))));
     }
   }
-  drupal_add_css(drupal_get_path('module', 'update') . '/update.css');
+  drupal_add_css(drupal_get_path('module', 'update') . '/css/update.admin.css');
   return $output;
 }
 
diff --git a/core/modules/user/user-rtl.css b/core/modules/user/css/user.module-rtl.css
similarity index 100%
rename from core/modules/user/user-rtl.css
rename to core/modules/user/css/user.module-rtl.css
diff --git a/core/modules/user/user.css b/core/modules/user/css/user.module.css
similarity index 94%
rename from core/modules/user/user.css
rename to core/modules/user/css/user.module.css
index 2c88121..ce8b166 100644
--- a/core/modules/user/user.css
+++ b/core/modules/user/css/user.module.css
@@ -94,9 +94,9 @@ div.password-suggestions ul {
  * Toolbar icon.
  */
 .icon-user:before {
-  background-image: url("images/icon-user.png");
+  background-image: url(../images/icon-user.png);
 }
 .icon-user:active:before,
 .icon-user.active:before {
-  background-image: url("images/icon-user-active.png");
+  background-image: url(../images/icon-user-active.png);
 }
diff --git a/core/modules/user/user.module b/core/modules/user/user.module
index abfe84b..56988ff 100644
--- a/core/modules/user/user.module
+++ b/core/modules/user/user.module
@@ -118,7 +118,7 @@ function user_theme() {
  */
 function user_page_build(&$page) {
   $path = drupal_get_path('module', 'user');
-  $page['#attached']['css'][$path . '/user.css'] = array('every_page' => TRUE);
+  $page['#attached']['css'][$path . '/css/user.module.css'] = array('every_page' => TRUE);
 }
 
 /**
@@ -2761,7 +2761,7 @@ function user_library_info() {
       drupal_get_path('module', 'user') . '/user.js' => array(),
     ),
     'css' => array(
-      drupal_get_path('module', 'user') . '/user.css' => array(),
+      drupal_get_path('module', 'user') . '/css/user.module.css' => array(),
     ),
     'dependencies' => array(
       array('system', 'jquery'),
diff --git a/core/modules/views/css/views.base.css b/core/modules/views/css/views.module.css
similarity index 100%
rename from core/modules/views/css/views.base.css
rename to core/modules/views/css/views.module.css
diff --git a/core/modules/views/includes/ajax.inc b/core/modules/views/includes/ajax.inc
index d938e34..5729f74 100644
--- a/core/modules/views/includes/ajax.inc
+++ b/core/modules/views/includes/ajax.inc
@@ -108,7 +108,7 @@ function views_ajax_form_wrapper($form_id, &$form_state) {
   // These forms have the title built in, so set the title here:
   if (empty($form_state['ajax']) && !empty($form_state['title'])) {
     drupal_set_title($form_state['title']);
-    drupal_add_css(drupal_get_path('module', 'views_ui') . '/css/views-admin.css');
+    drupal_add_css(drupal_get_path('module', 'views_ui') . '/css/views_ui.admin.css');
   }
 
   if (!empty($form_state['ajax']) && (empty($form_state['executed']) || !empty($form_state['rerender']))) {
diff --git a/core/modules/views/lib/Drupal/views/Tests/Plugin/CacheTest.php b/core/modules/views/lib/Drupal/views/Tests/Plugin/CacheTest.php
index 886d815..f1da997 100644
--- a/core/modules/views/lib/Drupal/views/Tests/Plugin/CacheTest.php
+++ b/core/modules/views/lib/Drupal/views/Tests/Plugin/CacheTest.php
@@ -162,7 +162,7 @@ function testHeaderStorage() {
     // Make sure that this css is not added when running the cached view.
     $view->storage->set('id', 'test_cache_header_storage_2');
 
-    $system_css_path = drupal_get_path('module', 'system') . '/system.maintenance.css';
+    $system_css_path = drupal_get_path('module', 'system') . '/css/system.maintenance.css';
     drupal_add_css($system_css_path);
     $system_js_path = drupal_get_path('module', 'system') . '/system.cron.js';
     drupal_add_js($system_js_path);
diff --git a/core/modules/views/lib/Drupal/views/ViewExecutable.php b/core/modules/views/lib/Drupal/views/ViewExecutable.php
index 9bef1d1..4e352fd 100644
--- a/core/modules/views/lib/Drupal/views/ViewExecutable.php
+++ b/core/modules/views/lib/Drupal/views/ViewExecutable.php
@@ -424,7 +424,7 @@ public function __construct(ViewStorageInterface $storage) {
     $this->storage->set('executable', $this);
 
     // Add the default css for a view.
-    $this->element['#attached']['css'][] = drupal_get_path('module', 'views') . '/css/views.base.css';
+    $this->element['#attached']['css'][] = drupal_get_path('module', 'views') . '/css/views.module.css';
   }
 
   /**
diff --git a/core/modules/views_ui/css/views-admin-rtl.css b/core/modules/views_ui/css/views_ui.admin-rtl.css
similarity index 100%
rename from core/modules/views_ui/css/views-admin-rtl.css
rename to core/modules/views_ui/css/views_ui.admin-rtl.css
diff --git a/core/modules/views_ui/css/views-admin.css b/core/modules/views_ui/css/views_ui.admin.css
similarity index 97%
rename from core/modules/views_ui/css/views-admin.css
rename to core/modules/views_ui/css/views_ui.admin.css
index 64b8070..c0a37566 100644
--- a/core/modules/views_ui/css/views-admin.css
+++ b/core/modules/views_ui/css/views_ui.admin.css
@@ -1,6 +1,6 @@
 /**
- * The .css file is intended to only contain positioning and size declarations
- * For example: display, position, float, clear, and overflow.
+ * The .admin.css file is intended to only contain positioning and size
+ * declarations. For example: display, position, float, clear, and overflow.
  */
 
 /* @group Resets */
diff --git a/core/modules/views_ui/css/views-admin.theme-rtl.css b/core/modules/views_ui/css/views_ui.admin.theme-rtl.css
similarity index 96%
rename from core/modules/views_ui/css/views-admin.theme-rtl.css
rename to core/modules/views_ui/css/views_ui.admin.theme-rtl.css
index c4c89b1..6548a17 100644
--- a/core/modules/views_ui/css/views-admin.theme-rtl.css
+++ b/core/modules/views_ui/css/views_ui.admin.theme-rtl.css
@@ -1,6 +1,6 @@
 /**
- * The .theme.css file is intended to contain presentation declarations including
- * images, borders, colors, and fonts.
+ * The .admin.theme.css file is intended to contain presentation declarations
+ * including images, borders, colors, and fonts.
  */
 
 /* @end */
diff --git a/core/modules/views_ui/css/views-admin.theme.css b/core/modules/views_ui/css/views_ui.admin.theme.css
similarity index 99%
rename from core/modules/views_ui/css/views-admin.theme.css
rename to core/modules/views_ui/css/views_ui.admin.theme.css
index 59bed90..bce3fdc 100644
--- a/core/modules/views_ui/css/views-admin.theme.css
+++ b/core/modules/views_ui/css/views_ui.admin.theme.css
@@ -1,6 +1,6 @@
 /**
- * The .theme.css file is intended to contain presentation declarations including
- * images, borders, colors, and fonts.
+ * The .admin.theme.css file is intended to contain presentation declarations
+ * including images, borders, colors, and fonts.
  */
 
 /* @group Reset */
diff --git a/core/modules/views_ui/css/views-admin.contextual.css b/core/modules/views_ui/css/views_ui.contextual.css
similarity index 100%
rename from core/modules/views_ui/css/views-admin.contextual.css
rename to core/modules/views_ui/css/views_ui.contextual.css
diff --git a/core/modules/views_ui/lib/Drupal/views_ui/ViewFormControllerBase.php b/core/modules/views_ui/lib/Drupal/views_ui/ViewFormControllerBase.php
index c0fe439..98637f7 100644
--- a/core/modules/views_ui/lib/Drupal/views_ui/ViewFormControllerBase.php
+++ b/core/modules/views_ui/lib/Drupal/views_ui/ViewFormControllerBase.php
@@ -79,11 +79,11 @@ protected function prepareEntity() {
   public static function getAdminCSS() {
     $module_path = drupal_get_path('module', 'views_ui');
     $list = array();
-    $list[$module_path . '/css/views-admin.css'] = array();
-    $list[$module_path . '/css/views-admin.theme.css'] = array();
+    $list[$module_path . '/css/views_ui.admin.css'] = array();
+    $list[$module_path . '/css/views_ui.admin.theme.css'] = array();
 
     if (\Drupal::moduleHandler()->moduleExists('contextual')) {
-      $list[$module_path . '/css/views-admin.contextual.css'] = array();
+      $list[$module_path . '/css/views_ui.contextual.css'] = array();
     }
 
     return $list;
diff --git a/core/themes/seven/seven.theme b/core/themes/seven/seven.theme
index 0360b6e..dbceaac 100644
--- a/core/themes/seven/seven.theme
+++ b/core/themes/seven/seven.theme
@@ -145,7 +145,7 @@ function seven_form_node_form_alter(&$form, &$form_state) {
 
   $form['#theme'] = array('node_edit_form');
   $form['#attached'] = array(
-    'css' => array(drupal_get_path('module', 'node') . '/node.edit.admin.css'),
+    'css' => array(drupal_get_path('module', 'node') . '/css/node.module.css'),
   );
 
   $form['advanced']['#type'] = 'container';
diff --git a/core/themes/seven/style.css b/core/themes/seven/style.css
index 4dfa8b4..9611ad6 100644
--- a/core/themes/seven/style.css
+++ b/core/themes/seven/style.css
@@ -1552,7 +1552,7 @@ details.fieldset-no-legend {
  * Widescreen
  *
  * Both of the following media queries must *exactly* match what is in
- * node.edit.admin.css. This is rather crazy.
+ * node.module.css. This is rather crazy.
  *
  * @todo Figure out how to reduce media query duplication across files
  *       and modules. Layout styles only allowed in themes?