core/includes/theme.inc                                |   16 ++++++++++++++++
 .../Drupal/Core/Page/DefaultHtmlFragmentRenderer.php   |    5 +++--
 core/modules/block/lib/Drupal/block/Entity/Block.php   |    3 +--
 .../modules/block/lib/Drupal/block/Tests/BlockTest.php |    6 ++++--
 .../menu/lib/Drupal/menu/Tests/MenuCacheTagsTest.php   |    5 +++--
 .../lib/Drupal/system/Form/ThemeSettingsForm.php       |   10 ++++++++++
 .../Drupal/system/Tests/Bootstrap/PageCacheTest.php    |    9 ++++++++-
 .../Tests/Cache/PageCacheTagsIntegrationTest.php       |    2 ++
 .../system/Tests/Entity/EntityCacheTagsTestBase.php    |   15 ++++++++-------
 core/modules/system/system.module                      |    1 +
 10 files changed, 56 insertions(+), 16 deletions(-)

diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 178702b..ac6d77f 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1925,6 +1925,22 @@ function drupal_pre_render_html(array $element) {
 
   return $element;
 }
+
+/**
+ * #pre_render callback for the page element type.
+ *
+ * @param array $element
+ *   A structured array containing the page element type build properties.
+ *
+ * @see system_element_info()
+ */
+function drupal_pre_render_page(array $element) {
+  global $theme;
+  $element['#cache']['tags']['theme'] = $theme;
+  $element['#cache']['tags']['theme_global_settings'] = TRUE;
+  return $element;
+}
+
 /**
  * Prepares variables for HTML document templates.
  *
diff --git a/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php b/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php
index 18f21f9..8de2067 100644
--- a/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php
+++ b/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php
@@ -36,7 +36,6 @@ public function __construct(LanguageManager $language_manager) {
    * {@inheritdoc}
    */
   public function render(HtmlFragment $fragment, $status_code = 200) {
-    global $theme;
     // Converts the given HTML fragment which represents the main content region
     // of the page into a render array.
     $page_content['main'] = array(
@@ -57,7 +56,9 @@ public function render(HtmlFragment $fragment, $status_code = 200) {
     $page->setContent(drupal_render($page_array));
     // Collect cache tags for all the content in all the regions on the page.
     $tags = $page_array['#cache']['tags'];
-    $tags['theme'] = $theme;
+    // Enforce the generic "content" cache tag on all pages.
+    // @todo Remove the "content" cache tag. @see https://drupal.org/node/2124957
+    $tags['content'] = TRUE;
     $page->setCacheTags($tags);
     $page->setStatusCode($status_code);
 
diff --git a/core/modules/block/lib/Drupal/block/Entity/Block.php b/core/modules/block/lib/Drupal/block/Entity/Block.php
index 6baeebd..aa9141c 100644
--- a/core/modules/block/lib/Drupal/block/Entity/Block.php
+++ b/core/modules/block/lib/Drupal/block/Entity/Block.php
@@ -152,7 +152,6 @@ public function toArray() {
    * {@inheritdoc}
    */
   public function postSave(EntityStorageInterface $storage, $update = TRUE) {
-    global $theme;
     parent::postSave($storage, $update);
 
     if ($update) {
@@ -161,7 +160,7 @@ public function postSave(EntityStorageInterface $storage, $update = TRUE) {
     // When placing a new block, invalidate all cache entries for this theme,
     // since any page that uses this theme might be affected.
     else {
-      Cache::invalidateTags(array('theme' => $theme));
+      Cache::invalidateTags(array('theme' => $this->theme));
     }
   }
 
diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockTest.php
index bee1a48..734c2be 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockTest.php
@@ -281,11 +281,12 @@ public function testBlockCacheTags() {
     $cid = sha1(implode(':', $cid_parts));
     $cache_entry = \Drupal::cache('render')->get($cid);
     $expected_cache_tags = array(
+      'theme:stark',
+      'theme_global_settings:1',
       'content:1',
       'block_view:1',
       'block:powered',
       'block_plugin:system_powered_by_block',
-      'theme:stark',
     );
     $this->assertIdentical($cache_entry->tags, $expected_cache_tags);
     $cache_entry = \Drupal::cache('render')->get('entity_view:block:powered:en:stark');
@@ -319,12 +320,13 @@ public function testBlockCacheTags() {
     $cid = sha1(implode(':', $cid_parts));
     $cache_entry = \Drupal::cache('render')->get($cid);
     $expected_cache_tags = array(
+      'theme:stark',
+      'theme_global_settings:1',
       'content:1',
       'block_view:1',
       'block:powered-2',
       'block:powered',
       'block_plugin:system_powered_by_block',
-      'theme:stark',
     );
     $this->assertEqual($cache_entry->tags, $expected_cache_tags);
     $expected_cache_tags = array(
diff --git a/core/modules/menu/lib/Drupal/menu/Tests/MenuCacheTagsTest.php b/core/modules/menu/lib/Drupal/menu/Tests/MenuCacheTagsTest.php
index 2b0dfcb..029bd00 100644
--- a/core/modules/menu/lib/Drupal/menu/Tests/MenuCacheTagsTest.php
+++ b/core/modules/menu/lib/Drupal/menu/Tests/MenuCacheTagsTest.php
@@ -59,12 +59,13 @@ public function testMenuBlock() {
 
     // Verify a cache hit, but also the presence of the correct cache tags.
     $expected_tags = array(
+      'theme:stark',
+      'theme_global_settings:1',
       'content:1',
       'block_view:1',
       'block:' . $block->id(),
       'block_plugin:system_menu_block__llama',
       'menu:llama',
-      'theme:stark',
     );
     $this->verifyPageCache($path, 'HIT', $expected_tags);
 
@@ -118,7 +119,7 @@ public function testMenuBlock() {
     $this->verifyPageCache($path, 'MISS');
 
     // Verify a cache hit.
-    $this->verifyPageCache($path, 'HIT', array('theme:stark'));
+    $this->verifyPageCache($path, 'HIT', array('content:1', 'theme:stark', 'theme_global_settings:1'));
   }
 
 }
diff --git a/core/modules/system/lib/Drupal/system/Form/ThemeSettingsForm.php b/core/modules/system/lib/Drupal/system/Form/ThemeSettingsForm.php
index 9459aad..8367a03 100644
--- a/core/modules/system/lib/Drupal/system/Form/ThemeSettingsForm.php
+++ b/core/modules/system/lib/Drupal/system/Form/ThemeSettingsForm.php
@@ -418,6 +418,16 @@ public function submitForm(array &$form, array &$form_state) {
 
     theme_settings_convert_to_config($values, $config)->save();
 
+    // Invalidate either the theme-specific cache tag or the global theme
+    // settings cache tag, depending on whose settings were actually changed.
+    if (isset($values['theme'])) {
+      Cache::invalidateTags(array('theme' => $values['bartik']));
+    }
+    else {
+      Cache::invalidateTags(array('theme_global_settings' => TRUE));
+    }
+
+    // @todo Remove this in https://drupal.org/node/2124957.
     Cache::invalidateTags(array('content' => TRUE));
   }
 
diff --git a/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php b/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
index 2d36baa..7f6b6a0 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
@@ -66,7 +66,14 @@ function testPageCacheTags() {
     $cid = sha1(implode(':', $cid_parts));
     $cache_entry = \Drupal::cache('render')->get($cid);
     sort($cache_entry->tags);
-    $this->assertIdentical($cache_entry->tags, array('pre_render:1', 'system_test_cache_tags_page:1', 'theme:stark'));
+    $expected_tags = array(
+      'content:1',
+      'pre_render:1',
+      'system_test_cache_tags_page:1',
+      'theme:stark',
+      'theme_global_settings:1',
+    );
+    $this->assertIdentical($cache_entry->tags, $expected_tags);
 
     Cache::invalidateTags($tags);
     $this->drupalGet($path);
diff --git a/core/modules/system/lib/Drupal/system/Tests/Cache/PageCacheTagsIntegrationTest.php b/core/modules/system/lib/Drupal/system/Tests/Cache/PageCacheTagsIntegrationTest.php
index 4b39bcb..f9f76ac 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Cache/PageCacheTagsIntegrationTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Cache/PageCacheTagsIntegrationTest.php
@@ -78,6 +78,7 @@ function testPageCacheTags() {
     $this->verifyPageCacheTags('node/' . $node_1->id(), array(
       'content:1',
       'theme:bartik',
+      'theme_global_settings:1',
       'block_view:1',
       'block:bartik_content',
       'block:bartik_tools',
@@ -102,6 +103,7 @@ function testPageCacheTags() {
     $this->verifyPageCacheTags('node/' . $node_2->id(), array(
       'content:1',
       'theme:bartik',
+      'theme_global_settings:1',
       'block_view:1',
       'block:bartik_content',
       'block:bartik_tools',
diff --git a/core/modules/system/lib/Drupal/system/Tests/Entity/EntityCacheTagsTestBase.php b/core/modules/system/lib/Drupal/system/Tests/Entity/EntityCacheTagsTestBase.php
index b6c25b1..5112cc2 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Entity/EntityCacheTagsTestBase.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Entity/EntityCacheTagsTestBase.php
@@ -210,6 +210,8 @@ public function testReferencedEntity() {
     $non_referencing_entity_path = $this->non_referencing_entity->getSystemPath();
     $listing_path = 'entity_test/list/' . $entity_type . '_reference/' . $entity_type . '/' . $this->entity->id();
 
+    $theme_cache_tags = array('content:1', 'theme:stark', 'theme_global_settings:1');
+
     // Generate the standardized entity cache tags.
     $cache_tag = $entity_type . ':' . $this->entity->id();
     $view_cache_tag = $entity_type . '_view:1';
@@ -232,7 +234,7 @@ public function testReferencedEntity() {
     $this->verifyPageCache($referencing_entity_path, 'MISS');
 
     // Verify a cache hit, but also the presence of the correct cache tags.
-    $tags = array_merge(array('theme:stark'), $referencing_entity_cache_tags);
+    $tags = array_merge($theme_cache_tags, $referencing_entity_cache_tags);
     $this->verifyPageCache($referencing_entity_path, 'HIT', $tags);
 
     // Also verify the existence of an entity render cache entry.
@@ -245,7 +247,7 @@ public function testReferencedEntity() {
     $this->verifyPageCache($non_referencing_entity_path, 'MISS');
 
     // Verify a cache hit, but also the presence of the correct cache tags.
-    $tags = array_merge(array('theme:stark'), $non_referencing_entity_cache_tags);
+    $tags = array_merge($theme_cache_tags, $non_referencing_entity_cache_tags);
     $this->verifyPageCache($non_referencing_entity_path, 'HIT', $tags);
 
     // Also verify the existence of an entity render cache entry.
@@ -259,7 +261,7 @@ public function testReferencedEntity() {
     $this->verifyPageCache($listing_path, 'MISS');
 
     // Verify a cache hit, but also the presence of the correct cache tags.
-    $tags = array_merge(array('theme:stark'), $referencing_entity_cache_tags);
+    $tags = array_merge($theme_cache_tags, $referencing_entity_cache_tags);
     $this->verifyPageCache($listing_path, 'HIT', $tags);
 
 
@@ -407,13 +409,12 @@ public function testReferencedEntity() {
     $this->verifyPageCache($non_referencing_entity_path, 'HIT');
 
     // Verify cache hits.
-    $tags = array(
+    $tags = array_merge($theme_cache_tags, array(
       'entity_test_view:1',
       'entity_test:' . $this->referencing_entity->id(),
-      'theme:stark',
-    );
+    ));
     $this->verifyPageCache($referencing_entity_path, 'HIT', $tags);
-    $this->verifyPageCache($listing_path, 'HIT', array('theme:stark'));
+    $this->verifyPageCache($listing_path, 'HIT', $theme_cache_tags);
   }
 
 }
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 7b35fca..e782dc7 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -294,6 +294,7 @@ function system_element_info() {
   );
   $types['page'] = array(
     '#show_messages' => TRUE,
+    '#pre_render' => array('drupal_pre_render_page'),
     '#theme' => 'page',
     '#title' => '',
   );