diff --git a/core/modules/block/block.routing.yml b/core/modules/block/block.routing.yml
index a2ca6e5..6e7e12d 100644
--- a/core/modules/block/block.routing.yml
+++ b/core/modules/block/block.routing.yml
@@ -24,6 +24,7 @@ block_admin_add:
   pattern: '/admin/structure/block/add/{plugin_id}/{theme}'
   defaults:
     _content: '\Drupal\block\Controller\BlockAddController::blockAddConfigureForm'
+    theme: null
     _title: 'Configure block'
   requirements:
     _permission: 'administer blocks'
diff --git a/core/modules/block/lib/Drupal/block/BlockFormController.php b/core/modules/block/lib/Drupal/block/BlockFormController.php
index e873888..ed8f28b 100644
--- a/core/modules/block/lib/Drupal/block/BlockFormController.php
+++ b/core/modules/block/lib/Drupal/block/BlockFormController.php
@@ -8,6 +8,7 @@
 namespace Drupal\block;
 
 use Drupal\Core\Cache\Cache;
+use Drupal\Core\Config\ConfigFactory;
 use Drupal\Core\Entity\EntityFormController;
 use Drupal\Core\Entity\EntityManager;
 use Drupal\Core\Entity\Query\QueryFactory;
@@ -49,6 +50,13 @@ class BlockFormController extends EntityFormController {
   protected $languageManager;
 
   /**
+   * The config factory.
+   *
+   * @var \Drupal\Core\Config\ConfigFactory
+   */
+  protected $configFactory;
+
+  /**
    * Constructs a BlockFormController object.
    *
    * @param \Drupal\Core\Entity\EntityManager $entity_manager
@@ -57,11 +65,14 @@ class BlockFormController extends EntityFormController {
    *   The entity query factory.
    * @param \Drupal\Core\Language\LanguageManager $language_manager
    *   The language manager.
+   * @param \Drupal\Core\Config\ConfigFactory $config_factory
+   *   The config factory.
    */
-  public function __construct(EntityManager $entity_manager, QueryFactory $entity_query_factory, LanguageManager $language_manager) {
+  public function __construct(EntityManager $entity_manager, QueryFactory $entity_query_factory, LanguageManager $language_manager, ConfigFactory $config_factory) {
     $this->storageController = $entity_manager->getStorageController('block');
     $this->entityQueryFactory = $entity_query_factory;
     $this->languageManager = $language_manager;
+    $this->configFactory = $config_factory;
   }
 
   /**
@@ -71,7 +82,8 @@ public static function create(ContainerInterface $container) {
     return new static(
       $container->get('entity.manager'),
       $container->get('entity.query'),
-      $container->get('language_manager')
+      $container->get('language_manager'),
+      $container->get('config.factory')
     );
   }
 
@@ -228,6 +240,32 @@ public function form(array $form, array &$form_state) {
       '#description' => $this->t('Show this block only for the selected role(s). If you select no roles, the block will be visible to all users.'),
     );
 
+    // Theme settings.
+    if ($theme = $entity->get('theme')) {
+      $form['theme'] = array(
+        '#type' => 'value',
+        '#value' => $entity->get('theme'),
+      );
+    }
+    else {
+      $theme_options = array();
+      foreach (list_themes() as $theme_name => $theme_info) {
+        if (!empty($theme_info->status)) {
+          $theme_options[$theme_name] = $theme_info->info['name'];
+        }
+      }
+      $theme = $this->configFactory->get('system.theme')->get('default');
+      $form['theme'] = array(
+        '#type' => 'select',
+        '#options' => $theme_options,
+        '#title' => t('Theme'),
+        '#default_value' => $theme,
+        '#ajax' => array(
+          'callback' => array($this, 'themeSwitch'),
+          'wrapper' => 'edit-block-region-wrapper',
+        ),
+      );
+    }
     // Region settings.
     $form['region'] = array(
       '#type' => 'select',
@@ -235,12 +273,22 @@ public function form(array $form, array &$form_state) {
       '#description' => $this->t('Select the region where this block should be displayed.'),
       '#default_value' => $entity->get('region'),
       '#empty_value' => BLOCK_REGION_NONE,
-      '#options' => system_region_list($entity->get('theme'), REGIONS_VISIBLE),
+      '#options' => system_region_list($theme, REGIONS_VISIBLE),
+      '#prefix' => '<div id="edit-block-region-wrapper">',
+      '#suffix' => '</div>',
     );
     return $form;
   }
 
   /**
+   * Handles switching the available regions based on the selected theme.
+   */
+  public function themeSwitch($form, &$form_state) {
+    $form['region']['#options'] = system_region_list($form_state['values']['theme'], REGIONS_VISIBLE);
+    return $form['region'];
+  }
+
+  /**
    * {@inheritdoc}
    */
   protected function actions(array $form, array &$form_state) {
@@ -257,7 +305,7 @@ public function validate(array $form, array &$form_state) {
 
     $entity = $this->entity;
     if ($entity->isNew()) {
-      form_set_value($form['id'], $entity->get('theme') . '.' . $form_state['values']['machine_name'], $form_state);
+      form_set_value($form['id'], $form_state['values']['theme'] . '.' . $form_state['values']['machine_name'], $form_state);
     }
     if (!empty($form['machine_name']['#disabled'])) {
       $config_id = explode('.', $form_state['values']['machine_name']);
@@ -293,7 +341,7 @@ public function submit(array $form, array &$form_state) {
 
     drupal_set_message($this->t('The block configuration has been saved.'));
     Cache::invalidateTags(array('content' => TRUE));
-    $form_state['redirect'] = array('admin/structure/block/list/' . $entity->get('theme'), array(
+    $form_state['redirect'] = array('admin/structure/block/list/' . $form_state['values']['theme'], array(
       'query' => array('block-placement' => drupal_html_class($this->entity->id())),
     ));
   }
diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockTest.php
index 94c8b9e..e90e407 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockTest.php
@@ -145,6 +145,31 @@ function testBlock() {
   }
 
   /**
+   * Tests that the block form has a theme selector when not passed via the URL.
+   */
+  public function testBlockThemeSelector() {
+    // Enable all themes.
+    theme_enable(array('bartik', 'seven'));
+    $theme_settings = $this->container->get('config.factory')->get('system.theme');
+    foreach (array('bartik', 'stark', 'seven') as $theme) {
+      // Select the 'Powered by Drupal' block to be placed.
+      $block = array();
+      $block['machine_name'] = strtolower($this->randomName(8));
+      $block['theme'] = $theme;
+      $block['region'] = 'content';
+      $this->drupalPost('admin/structure/block/add/system_powered_by_block', $block, t('Save block'));
+      $this->assertText(t('The block configuration has been saved.'));
+      $this->assertUrl('admin/structure/block/list/' . $theme . '?block-placement=' . drupal_html_class($theme . ':' . $block['machine_name']));
+
+      // Set the default theme and ensure the block is placed.
+      $theme_settings->set('default', $theme)->save();
+      $this->drupalGet('');
+      $elements = $this->xpath('//div[@id = :id]', array(':id' => drupal_html_id('block-' . $block['machine_name'])));
+      $this->assertTrue(!empty($elements), 'The block was found.');
+    }
+  }
+
+  /**
    * Test block title display settings.
    */
   function testHideBlockTitle() {
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index 1ebccca..5e32613 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -2377,7 +2377,11 @@ protected function assertUrl($path, array $options = array(), $message = '', $gr
       ));
     }
     $options['absolute'] = TRUE;
-    return $this->assertEqual($this->getUrl(), $this->container->get('url_generator')->generateFromPath($path, $options), $message, $group);
+    // Paths in query strings can be encoded or decoded with no functional
+    // difference, decode them for comparison purposes.
+    $actual_url = urldecode($this->getUrl());
+    $expected_url = urldecode($this->container->get('url_generator')->generateFromPath($path, $options));
+    return $this->assertEqual($actual_url, $expected_url, $message, $group);
   }
 
   /**