diff --git a/core/modules/layout/layout.module b/core/modules/layout/layout.module
index 7b0ab2c..ecb325f 100644
--- a/core/modules/layout/layout.module
+++ b/core/modules/layout/layout.module
@@ -37,3 +37,24 @@ function layout_page_build(&$page) {
 function layout_manager() {
   return drupal_container()->get('plugin.manager.layout');
 }
+
+/**
+ * Implements hook_theme().
+ *
+ * Expose all layouts as theme items, so themes can override layout markup.
+ */
+function layout_theme($existing, $type, $theme, $path) {
+  $items = array();
+  foreach (layout_manager()->getDefinitions() as $name => $layout) {
+    $regions = array();
+    foreach (array_keys($layout['regions']) as $key) {
+      $regions[$key] = '';
+    }
+    $items[$layout['theme']] = array(
+      'variables' => $regions,
+      'path' => $layout['path'],
+      'template' => $layout['template'],
+    );
+  }
+  return $items;
+}
diff --git a/core/modules/layout/lib/Drupal/layout/Plugin/Derivative/Layout.php b/core/modules/layout/lib/Drupal/layout/Plugin/Derivative/Layout.php
index ef603c3..7ee90bf 100644
--- a/core/modules/layout/lib/Drupal/layout/Plugin/Derivative/Layout.php
+++ b/core/modules/layout/lib/Drupal/layout/Plugin/Derivative/Layout.php
@@ -100,8 +100,10 @@ protected function iterateDirectories($dir, $provider) {
         // layout subdirectory. This provides all information about the layout
         // such as layout markup template and CSS and JavaScript files to use.
         $directory = new FileStorage($fileinfo->getPath());
-        $this->derivatives[$provider['provider'] . '__' .  $fileinfo->getBasename('.yml')] = $directory->read($fileinfo->getBasename('.yml'));
-        $this->derivatives[$provider['provider'] . '__' .  $fileinfo->getBasename('.yml')]['path'] = $fileinfo->getPath();
+        $key = $provider['provider'] . '__' .  $fileinfo->getBasename('.yml');
+        $this->derivatives[$key] = $directory->read($fileinfo->getBasename('.yml'));
+        $this->derivatives[$key]['theme'] = $key;
+        $this->derivatives[$key]['path'] = $fileinfo->getPath();
       }
     }
   }
diff --git a/core/modules/layout/lib/Drupal/layout/Plugin/layout/layout/StaticLayout.php b/core/modules/layout/lib/Drupal/layout/Plugin/layout/layout/StaticLayout.php
index 601e343..0f46e66 100644
--- a/core/modules/layout/lib/Drupal/layout/Plugin/layout/layout/StaticLayout.php
+++ b/core/modules/layout/lib/Drupal/layout/Plugin/layout/layout/StaticLayout.php
@@ -45,52 +45,47 @@ public function getRegions() {
   /**
    * Add the CSS files associated with this layout to the page.
    */
-  public function addCss() {
+  public function getCssFiles() {
     $definition = $this->getDefinition();
     $files = $definition['css'];
     if (is_string($files)) {
       $files = array($files);
     }
-    foreach ($files as $filename) {
-      drupal_add_css($definition['path'] . '/' . $filename);
-    }
+    return $files;
   }
 
   /**
    * Add the administrative CSS files associated with this layout to the page.
    */
-  public function addAdminCss() {
+  public function getAdminCssFiles() {
     $definition = $this->getDefinition();
     // Fall back on regular CSS for the admin page if admin CSS not provided.
     $files = isset($definition['admin css']) ? $definition['admin css'] : $definition['css'];
     if (is_string($files)) {
       $files = array($files);
     }
-    foreach ($files as $filename) {
-      drupal_add_css($definition['path'] . '/' . $filename);
-    }
+    return $files;
   }
 
   /**
    * Add the JS files associated with this layout to the page.
    */
-  public function addJs() {
+  public function getJsFiles() {
     $definition = $this->getDefinition();
     if (isset($definition['js']))  {
       $files = $definition['js'];
       if (is_string($files)) {
         $files = array($files);
       }
-      foreach ($files as $filename) {
-        drupal_add_js($definition['path'] . '/' . $filename);
-      }
+      return $files;
     }
+    return array();
   }
 
   /**
    * Add the admin JS files associated with this layout to the page.
    */
-  public function addAdminJs() {
+  public function getAdminJsFiles() {
     $definition = $this->getDefinition();
     $files = array();
     if (isset($definition['admin js'])) {
@@ -103,9 +98,7 @@ public function addAdminJs() {
     if (is_string($files)) {
       $files = array($files);
     }
-    foreach ($files as $filename) {
-      drupal_add_js($definition['path'] . '/' . $filename);
-    }
+    return $files;
   }
 
   /**
@@ -114,27 +107,40 @@ public function addAdminJs() {
   public function renderLayout($admin = FALSE) {
     $definition = $this->getDefinition();
 
+    // Assemble a render array with the regions and attached CSS/JS.
+    $render = array(
+      '#theme' => $definition['theme'],
+    );
+
     // Render all regions needed for this layout.
-    $regions = array();
     foreach ($this->getRegions() as $region => $title) {
       // @todo This is just stub code to fill in regions with stuff for now.
       // When blocks are related to layouts and not themes, we can make this
       // really be filled in with blocks.
-      $regions[$region] = '<h3>' . $title . '</h3>';
+      $render['#' . $region] = '<h3>' . $title . '</h3>';
     }
 
+    // Fill in attached CSS and JS files based on metadata.
     if (!$admin) {
-      $this->addCss();
-      $this->addJs();
+      $render['#attached'] = array(
+        'css' => $this->getCssFiles(),
+        'js' => $this->getJsFiles(),
+      );
     }
     else {
-      $this->addAdminCss();
-      $this->addAdminJs();
+      $render['#attached'] = array(
+        'css' => $this->getAdminCssFiles(),
+        'js' => $this->getAdminJsFiles(),
+      );
     }
 
-    $template = $definition['path'] . '/' . $definition['template'] . '.tpl.php';
-    $output = theme_render_template($template, array('content' => $regions));
+    // Include the path of the definition in all CSS and JS files.
+    foreach (array('css', 'js') as $type) {
+      foreach ($render['#attached'][$type] as &$filename) {
+        $filename = $definition['path'] . '/' . $filename;
+      }
+    }
 
-    return $output;
+    return drupal_render($render);
   }
 }