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..c7fb3cd 100644
--- a/core/modules/block/lib/Drupal/block/BlockFormController.php
+++ b/core/modules/block/lib/Drupal/block/BlockFormController.php
@@ -228,6 +228,32 @@ 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 = \Drupal::config('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 +261,22 @@ 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 +293,7 @@ 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 +329,7 @@ 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..13064c6 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockTest.php
@@ -145,6 +145,22 @@ function testBlock() {
   }
 
   /**
+   * Tests that the block form has a theme selector when not passed via the URL.
+   */
+  public function testBlockThemeSelector() {
+    // Select the 'Powered by Drupal' block to be configured and moved.
+    $block = array();
+    $block['machine_name'] = strtolower($this->randomName(8));
+    $block['theme'] = \Drupal::config('system.theme')->get('default');
+    $block['region'] = 'header';
+
+    $this->drupalPost('admin/structure/block/add/system_powered_by_block', $block, t('Save block'));
+    $this->assertText(t('The block configuration has been saved.'));
+    $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() {