diff --git a/core/includes/ajax.inc b/core/includes/ajax.inc
index d8eb913..212dff6 100644
--- a/core/includes/ajax.inc
+++ b/core/includes/ajax.inc
@@ -411,7 +411,7 @@ function ajax_base_page_theme() {
     // to see the default theme, token validation isn't required for that, and
     // bypassing it allows most use-cases to work even when accessed from the
     // page cache.
-    if ($theme === variable_get('theme_default', 'stark') || drupal_valid_token($token, $theme)) {
+    if ($theme === config('system.theme')->get('default') || drupal_valid_token($token, $theme)) {
       return $theme;
     }
   }
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index cc60af2..3051560 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -90,7 +90,7 @@ function drupal_theme_initialize() {
 
   // Only select the user selected theme if it is available in the
   // list of themes that can be accessed.
-  $theme = !empty($user->theme) && drupal_theme_access($user->theme) ? $user->theme : variable_get('theme_default', 'stark');
+  $theme = !empty($user->theme) && drupal_theme_access($user->theme) ? $user->theme : config('system.theme')->get('default');
 
   // Allow modules to override the theme. Validation has already been performed
   // inside menu_get_custom_theme(), so we do not need to check it again here.
@@ -1530,7 +1530,7 @@ function theme_enable($theme_list) {
  */
 function theme_disable($theme_list) {
   // Don't disable the default theme.
-  if ($pos = array_search(variable_get('theme_default', 'stark'), $theme_list) !== FALSE) {
+  if ($pos = array_search(config('system.theme')->get('default'), $theme_list) !== FALSE) {
     unset($theme_list[$pos]);
     if (empty($theme_list)) {
       return;
diff --git a/core/includes/theme.maintenance.inc b/core/includes/theme.maintenance.inc
index 0b426e2..a03722a 100644
--- a/core/includes/theme.maintenance.inc
+++ b/core/includes/theme.maintenance.inc
@@ -50,7 +50,7 @@ function _drupal_maintenance_theme() {
     //   Stark otherwise. Since there is no low-level access to configuration
     //   currently, we only consult settings.php and fall back to Bartik
     //   otherwise, as it looks generic enough and way more user-friendly.
-    $custom_theme = variable_get('maintenance_theme', variable_get('theme_default', 'bartik'));
+    $custom_theme = variable_get('maintenance_theme', config('system.theme')->get('default')) ?: 'bartik';
   }
 
   // Ensure that system.module is loaded.
diff --git a/core/modules/block/block.admin.inc b/core/modules/block/block.admin.inc
index e5e8d76..eab2d8d 100644
--- a/core/modules/block/block.admin.inc
+++ b/core/modules/block/block.admin.inc
@@ -70,7 +70,7 @@ function block_admin_edit(Block $entity) {
   $theme = $themes[$theme_key];
   // Use meaningful titles for the main site and administrative themes.
   $theme_title = $theme->info['name'];
-  if ($theme_key == variable_get('theme_default', 'stark')) {
+  if ($theme_key == config('system.theme')->get('default')) {
     $theme_title = t('!theme (default theme)', array('!theme' => $theme_title));
   }
   elseif ($admin_theme && $theme_key == $admin_theme) {
diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index ff804b2..97ad2d2 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -59,7 +59,7 @@ function block_help($path, $arg) {
       $output .= '<dd>' . t('Blocks can be configured to be visible only on certain pages, only to users of certain roles, or only on pages displaying certain <a href="@content-type">content types</a>. Some dynamic blocks, such as those generated by modules, will be displayed only on certain pages.', array('@content-type' => url('admin/structure/types'), '@user' => url('user'))) . '</dd>';
       if (module_exists('custom_block')) {
         $output .= '<dt>' . t('Creating custom blocks') . '</dt>';
-        $output .= '<dd>' . t('Users with the <em>Administer blocks</em> permission can <a href="@block-add">add custom blocks</a>, which are then listed on the <a href="@blocks">Blocks administration page</a>. Once created, custom blocks behave just like default and module-generated blocks.', array('@blocks' => url('admin/structure/block'), '@block-add' => url('admin/structure/block/list/block_plugin_ui:' . variable_get('theme_default', 'stark') . '/add/custom_blocks'))) . '</dd>';
+        $output .= '<dd>' . t('Users with the <em>Administer blocks</em> permission can <a href="@block-add">add custom blocks</a>, which are then listed on the <a href="@blocks">Blocks administration page</a>. Once created, custom blocks behave just like default and module-generated blocks.', array('@blocks' => url('admin/structure/block'), '@block-add' => url('admin/structure/block/list/block_plugin_ui:' . config('system.theme')->get('default') . '/add/custom_blocks'))) . '</dd>';
       }
       $output .= '</dl>';
       return $output;
@@ -69,7 +69,7 @@ function block_help($path, $arg) {
       list(, $demo_theme) = explode(':', $arg[4]);
     }
     else {
-      $demo_theme = variable_get('theme_default', 'stark');
+      $demo_theme = config('system.theme')->get('default');
     }
     $themes = list_themes();
     $output = '<p>' . t('This page provides a drag-and-drop interface for assigning a block to a region, and for controlling the order of blocks within regions. Since not all themes implement the same regions, or display regions in the same way, blocks are positioned on a per-theme basis. Remember that your changes will not be saved until you click the <em>Save blocks</em> button at the bottom of the page. Click the <em>configure</em> link next to each block to configure its specific title and visibility settings.') . '</p>';
@@ -112,7 +112,7 @@ function block_permission() {
  * @todo Clarify the documentation for the per-plugin block admin links.
  */
 function block_menu() {
-  $default_theme = variable_get('theme_default', 'stark');
+  $default_theme = config('system.theme')->get('default');
   $items['admin/structure/block'] = array(
     'title' => 'Blocks',
     'description' => 'Configure what block content appears in your site\'s sidebars and other regions.',
@@ -267,7 +267,7 @@ function block_page_build(&$page) {
       $page['page_top']['backlink'] = array(
         '#type' => 'link',
         '#title' => t('Exit block region demonstration'),
-        '#href' => 'admin/structure/block' . (variable_get('theme_default', 'stark') == $theme ? '' : '/list/' . $theme),
+        '#href' => 'admin/structure/block' . (config('system.theme')->get('default') == $theme ? '' : '/list/' . $theme),
         // Add the "overlay-restore" class to indicate this link should restore
         // the context in which the region demonstration page was opened.
         '#options' => array('attributes' => array('class' => array('block-demo-backlink', 'overlay-restore'))),
@@ -363,7 +363,7 @@ function _block_get_renderable_region($list = array()) {
  *   Blocks currently exported by modules.
  */
 function _block_rehash($theme = NULL) {
-  $theme = $theme ? $theme : variable_get('theme_default', 'stark');
+  $theme = $theme ? $theme : config('system.theme')->get('default');
   $regions = system_region_list($theme);
   $blocks = entity_load_multiple_by_properties('block', array('theme' => $theme));
   foreach ($blocks as $block_id => $block) {
@@ -412,7 +412,7 @@ function block_theme_initialize($theme) {
   // Initialize theme's blocks if none already registered.
   $has_blocks = entity_load_multiple_by_properties('block', array('theme' => $theme));
   if (!$has_blocks) {
-    $default_theme = variable_get('theme_default', 'stark');
+    $default_theme = config('system.theme')->get('default');
     // Apply only to new theme's visible regions.
     $regions = system_region_list($theme, REGIONS_VISIBLE);
     $default_theme_blocks = entity_load_multiple_by_properties('block', array('theme' => $default_theme));
diff --git a/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockFormController.php b/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockFormController.php
index ccc7eca..0094c42 100644
--- a/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockFormController.php
+++ b/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockFormController.php
@@ -182,7 +182,7 @@ public function save(array $form, array &$form_state) {
           $form_state['redirect'] = 'admin/structure/block/add/custom_block:' . $block->uuid->value . '/' . $theme;
         }
         else {
-          $form_state['redirect'] = 'admin/structure/block/add/custom_block:' . $block->uuid->value . '/' . variable_get('theme_default', 'stark');
+          $form_state['redirect'] = 'admin/structure/block/add/custom_block:' . $block->uuid->value . '/' . config('system.theme')->get('default');
         }
       }
       else {
diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockHiddenRegionTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockHiddenRegionTest.php
index c16439c..1232420 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockHiddenRegionTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockHiddenRegionTest.php
@@ -61,7 +61,9 @@ public function testBlockNotInHiddenRegion() {
     // Enable "block_test_theme" and set it as the default theme.
     $theme = 'block_test_theme';
     theme_enable(array($theme));
-    variable_set('theme_default', $theme);
+    config('system.theme')
+      ->set('default', $theme)
+      ->save();
     menu_router_rebuild();
 
     // Ensure that "block_test_theme" is set as the default theme.
diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockLanguageTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockLanguageTest.php
index 16531df..f247648 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockLanguageTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockLanguageTest.php
@@ -54,7 +54,7 @@ function setUp() {
    */
   public function testLanguageBlockVisibility() {
     // Check if the visibility setting is available.
-    $default_theme = variable_get('theme_default', 'stark');
+    $default_theme = config('system.theme')->get('default');
     $this->drupalGet('admin/structure/block/add/system_powered_by_block' . '/' . $default_theme);
 
     $this->assertField('visibility[language][langcodes][en]', 'Language visibility field is visible.');
diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockTemplateSuggestionsUnitTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockTemplateSuggestionsUnitTest.php
index c469eb6..d707c80 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockTemplateSuggestionsUnitTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockTemplateSuggestionsUnitTest.php
@@ -40,7 +40,7 @@ function testBlockThemeHookSuggestions() {
     $block = entity_create('block', array(
       'plugin' => 'system_menu_block:menu-admin',
       'region' => 'footer',
-      'id' => variable_get('theme_default', 'stark') . '.machinename',
+      'id' => config('system.theme')->get('default') . '.machinename',
     ));
 
     $variables = array();
diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockTest.php
index 38a0d29..60be778 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockTest.php
@@ -65,7 +65,7 @@ function setUp() {
    * Test creating custom block, moving it to a specific region and then deleting it.
    */
   public function testCustomBlock() {
-    $default_theme = variable_get('theme_default', 'stark');
+    $default_theme = config('system.theme')->get('default');
 
     // Clear the block cache to load the Custom Block module's block definitions.
     $this->container->get('plugin.manager.block')->clearCachedDefinitions();
@@ -150,7 +150,7 @@ public function testCustomBlock() {
    * Test creating custom block using Full HTML.
    */
   public function testCustomBlockFormat() {
-    $default_theme = variable_get('theme_default', 'stark');
+    $default_theme = config('system.theme')->get('default');
 
     // Add a new custom block by filling out the input form on block/add/basic.
     $info = strtolower($this->randomName(8));
@@ -200,7 +200,7 @@ function testBlockVisibility() {
     // Create a random title for the block.
     $title = $this->randomName(8);
     // Enable a standard block.
-    $default_theme = variable_get('theme_default', 'stark');
+    $default_theme = config('system.theme')->get('default');
     $edit = array(
       'machine_name' => strtolower($this->randomName(8)),
       'region' => 'sidebar_first',
@@ -240,7 +240,7 @@ function testBlockVisibilityListedEmpty() {
     // Create a random title for the block.
     $title = $this->randomName(8);
     // Enable a standard block.
-    $default_theme = variable_get('theme_default', 'stark');
+    $default_theme = config('system.theme')->get('default');
     $edit = array(
       'machine_name' => strtolower($this->randomName(8)),
       'region' => 'sidebar_first',
@@ -273,7 +273,7 @@ function testBlock() {
     $block['id'] = 'system_powered_by_block';
     $block['label'] = $this->randomName(8);
     $block['machine_name'] = strtolower($this->randomName(8));
-    $block['theme'] = variable_get('theme_default', 'stark');
+    $block['theme'] = config('system.theme')->get('default');
     $block['region'] = 'header';
 
     // Set block title to confirm that interface works and override any custom titles.
@@ -320,7 +320,7 @@ function testBlock() {
    */
   function moveBlockToRegion(array $block, $region) {
     // Set the created block to a specific region.
-    $block += array('theme' => variable_get('theme_default', 'stark'));
+    $block += array('theme' => config('system.theme')->get('default'));
     $edit = array();
     $edit['blocks[' . $block['theme'] . '.' . $block['machine_name'] . '][region]'] = $region;
     $this->drupalPost('admin/structure/block', $edit, t('Save blocks'));
@@ -354,7 +354,7 @@ function testBlockRehash() {
     $block = array();
     $block['id'] = 'test_cache';
     $block['machine_name'] = strtolower($this->randomName(8));
-    $block['theme'] = variable_get('theme_default', 'stark');
+    $block['theme'] = config('system.theme')->get('default');
     $block['region'] = 'header';
     $block = $this->drupalPlaceBlock('test_cache', array('region' => 'header'));
 
@@ -413,7 +413,7 @@ function testBlockModuleDisable() {
     }
 
     // Ensure that the disabled module's block plugin is no longer available.
-    $this->drupalGet('admin/structure/block/list/block_plugin_ui:' . variable_get('theme_default', 'stark') . '/add');
+    $this->drupalGet('admin/structure/block/list/block_plugin_ui:' . config('system.theme')->get('default') . '/add');
     $this->assertNoText(t('Test block caching'));
 
     // Confirm that the block is no longer displayed on the front page.
diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockTitleXSSTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockTitleXSSTest.php
index 41f7a14..b04f9fc 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockTitleXSSTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockTitleXSSTest.php
@@ -47,7 +47,7 @@ function testXSSInTitle() {
     $this->assertNoRaw('<script>alert("XSS label");</script>', 'The block title was properly sanitized when rendered.');
 
     $this->drupalLogin($this->drupalCreateUser(array('administer blocks', 'access administration pages')));
-    $default_theme = variable_get('theme_default', 'stark');
+    $default_theme = config('system.theme')->get('default');
     $this->drupalGet('admin/structure/block/list/block_plugin_ui:' . $default_theme . '/add');
     $this->assertNoRaw("<script>alert('XSS subject');</script>", 'The block title was properly sanitized in Block Plugin UI Admin page.');
   }
diff --git a/core/modules/block/lib/Drupal/block/Tests/NewDefaultThemeBlocksTest.php b/core/modules/block/lib/Drupal/block/Tests/NewDefaultThemeBlocksTest.php
index 190570c..f9a7792 100644
--- a/core/modules/block/lib/Drupal/block/Tests/NewDefaultThemeBlocksTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/NewDefaultThemeBlocksTest.php
@@ -33,7 +33,7 @@ public static function getInfo() {
    * Check the enabled Bartik blocks are correctly copied over.
    */
   function testNewDefaultThemeBlocks() {
-    $default_theme = variable_get('theme_default', 'stark');
+    $default_theme = config('system.theme')->get('default');
 
     // Add several block instances.
     $this->adminUser = $this->drupalCreateUser(array('administer blocks'));
@@ -51,7 +51,9 @@ function testNewDefaultThemeBlocks() {
     $new_theme = 'bartik';
     $this->assertFalse($new_theme == $default_theme, 'The new theme is different from the previous default theme.');
     theme_enable(array($new_theme));
-    variable_set('theme_default', $new_theme);
+    config('system.theme')
+      ->set('default', $new_theme)
+      ->save();
 
     // Ensure that the new theme has all the blocks as the previous default.
     // @todo Replace the string manipulation below once the configuration
diff --git a/core/modules/block/lib/Drupal/block/Tests/Views/DisplayBlockTest.php b/core/modules/block/lib/Drupal/block/Tests/Views/DisplayBlockTest.php
index b91bdcf..5f3b776 100644
--- a/core/modules/block/lib/Drupal/block/Tests/Views/DisplayBlockTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/Views/DisplayBlockTest.php
@@ -142,7 +142,7 @@ protected function testDeleteBlockDisplay() {
    */
   public function testViewsBlockForm() {
     $this->drupalLogin($this->drupalCreateUser(array('administer blocks')));
-    $default_theme = variable_get('theme_default', 'stark');
+    $default_theme = config('system.theme')->get('default');
     $this->drupalGet('admin/structure/block/add/views_block:test_view_block-block_1/' . $default_theme);
     $elements = $this->xpath('//input[@name="label"]');
     $this->assertTrue(empty($elements), 'The label field is not found for Views blocks.');
diff --git a/core/modules/ckeditor/ckeditor.module b/core/modules/ckeditor/ckeditor.module
index c566f3a..2b1358f 100644
--- a/core/modules/ckeditor/ckeditor.module
+++ b/core/modules/ckeditor/ckeditor.module
@@ -97,7 +97,7 @@ function ckeditor_theme() {
 function _ckeditor_theme_css($theme = NULL) {
   $css = array();
   if (!isset($theme)) {
-    $theme = variable_get('theme_default');
+    $theme = config('system.theme')->get('default');
   }
   if ($theme_path = drupal_get_path('theme', $theme)) {
     $info = system_get_info('theme', $theme);
diff --git a/core/modules/color/lib/Drupal/color/Tests/ColorTest.php b/core/modules/color/lib/Drupal/color/Tests/ColorTest.php
index 6720e04..fea5686 100644
--- a/core/modules/color/lib/Drupal/color/Tests/ColorTest.php
+++ b/core/modules/color/lib/Drupal/color/Tests/ColorTest.php
@@ -76,7 +76,9 @@ function testColor() {
    * Tests the Color module functionality using the given theme.
    */
   function _testColor($theme, $test_values) {
-    variable_set('theme_default', $theme);
+    config('system.theme')
+      ->set('default', $theme)
+      ->save();
     $settings_path = 'admin/appearance/settings/' . $theme;
 
     $this->drupalLogin($this->big_user);
@@ -122,7 +124,9 @@ function _testColor($theme, $test_values) {
    * Tests whether the provided color is valid.
    */
   function testValidColor() {
-    variable_set('theme_default', 'bartik');
+    config('system.theme')
+      ->set('default', 'bartik')
+      ->save();
     $settings_path = 'admin/appearance/settings/bartik';
 
     $this->drupalLogin($this->big_user);
diff --git a/core/modules/comment/lib/Drupal/comment/Tests/CommentLinksTest.php b/core/modules/comment/lib/Drupal/comment/Tests/CommentLinksTest.php
index 91cab48..8deb4e3 100644
--- a/core/modules/comment/lib/Drupal/comment/Tests/CommentLinksTest.php
+++ b/core/modules/comment/lib/Drupal/comment/Tests/CommentLinksTest.php
@@ -35,7 +35,9 @@ public static function getInfo() {
   function testCommentLinks() {
     // Bartik theme alters comment links, so use a different theme.
     theme_enable(array('stark'));
-    variable_set('theme_default', 'stark');
+    config('system.theme')
+      ->set('default', 'stark')
+      ->save();
 
     // Remove additional user permissions from $this->web_user added by setUp(),
     // since this test is limited to anonymous and authenticated roles only.
diff --git a/core/modules/menu/lib/Drupal/menu/Tests/MenuTest.php b/core/modules/menu/lib/Drupal/menu/Tests/MenuTest.php
index 88283cc..9f4dcc6 100644
--- a/core/modules/menu/lib/Drupal/menu/Tests/MenuTest.php
+++ b/core/modules/menu/lib/Drupal/menu/Tests/MenuTest.php
@@ -157,7 +157,7 @@ function addCustomMenu() {
     // Enable the custom menu block.
     $menu_name = 'menu-' . $menu_name; // Drupal prepends the name with 'menu-'.
     // Confirm that the custom menu block is available.
-    $this->drupalGet('admin/structure/block/list/block_plugin_ui:' . variable_get('theme_default', 'stark') . '/add');
+    $this->drupalGet('admin/structure/block/list/block_plugin_ui:' . config('system.theme')->get('default') . '/add');
     $this->assertText($label);
 
     // Enable the block.
diff --git a/core/modules/php/php.module b/core/modules/php/php.module
index 8e885e4..83f0b66 100644
--- a/core/modules/php/php.module
+++ b/core/modules/php/php.module
@@ -61,16 +61,16 @@ function php_permission() {
  * @see php_filter_info()
  */
 function php_eval($code) {
-  global $theme_path, $theme_info, $conf;
+  global $theme_path, $theme_info;
 
   // Store current theme path.
   $old_theme_path = $theme_path;
 
   // Restore theme_path to the theme, as long as php_eval() executes,
   // so code evaluated will not see the caller module as the current theme.
-  // If theme info is not initialized get the path from theme_default.
+  // If theme info is not initialized get the path from default theme.
   if (!isset($theme_info)) {
-    $theme_path = drupal_get_path('theme', $conf['theme_default']);
+    $theme_path = drupal_get_path('theme', config('system.theme')->get('default'));
   }
   else {
     $theme_path = dirname($theme_info->filename);
diff --git a/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php b/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
index fc953e5..ca1d88b 100644
--- a/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
+++ b/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
@@ -136,7 +136,9 @@ function testShortcutLinkDelete() {
    */
   function testNoShortcutLink() {
     // Change to a theme that displays shortcuts.
-    variable_set('theme_default', 'seven');
+    config('system.theme')
+      ->set('default', 'seven')
+      ->save();
 
     $this->drupalGet('page-that-does-not-exist');
     $this->assertNoRaw('add-shortcut', 'Add to shortcuts link was not shown on a page not found.');
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
index 6dbf439..bc472c4 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
@@ -119,6 +119,8 @@ protected function setUp() {
     $modules = array_reverse($modules);
     $modules = call_user_func_array('array_merge_recursive', $modules);
     $this->enableModules($modules, FALSE);
+    // In order to use theme functions default theme config needs to exist.
+    config('system.theme')->set('default', 'stark');
   }
 
   protected function tearDown() {
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index dce2ceb..4aa9032 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -376,7 +376,7 @@ protected function drupalPlaceBlock($plugin_id, array $values = array(), array $
       'plugin' => $plugin_id,
       'label' => $this->randomName(8),
       'region' => 'sidebar_first',
-      'theme' => variable_get('theme_default', 'stark'),
+      'theme' => config('system.theme')->get('default'),
       'machine_name' => strtolower($this->randomName(8)),
       'settings' => $settings,
     );
diff --git a/core/modules/system/config/system.theme.yml b/core/modules/system/config/system.theme.yml
index cf330e1..c56c7f2 100644
--- a/core/modules/system/config/system.theme.yml
+++ b/core/modules/system/config/system.theme.yml
@@ -1,3 +1,4 @@
 admin: '0'
 enabled:
   stark: '0'
+default: stark
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 e7aa0e0..f8944f9 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Ajax/FrameworkTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Ajax/FrameworkTest.php
@@ -157,7 +157,9 @@ function testLazyLoadOverriddenCSS() {
     // The test theme overrides system.base.css without an implementation,
     // thereby removing it.
     theme_enable(array('test_theme'));
-    variable_set('theme_default', 'test_theme');
+    config('system.theme')
+      ->set('default', 'test_theme')
+      ->save();
 
     // This gets the form, and emulates an Ajax submission on it, including
     // adding markup to the HEAD and BODY for any lazy loaded JS/CSS files.
diff --git a/core/modules/system/lib/Drupal/system/Tests/Batch/PageTest.php b/core/modules/system/lib/Drupal/system/Tests/Batch/PageTest.php
index 97593f2..be6bcef 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Batch/PageTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Batch/PageTest.php
@@ -35,7 +35,9 @@ public static function getInfo() {
   function testBatchProgressPageTheme() {
     // Make sure that the page which starts the batch (an administrative page)
     // is using a different theme than would normally be used by the batch API.
-    variable_set('theme_default', 'bartik');
+    config('system.theme')
+      ->set('default', 'bartik')
+      ->save();
     theme_enable(array('seven'));
     config('system.theme')->set('admin', 'seven')->save();
     // Log in as an administrator who can see the administrative theme.
diff --git a/core/modules/system/lib/Drupal/system/Tests/Menu/MenuRouterTest.php b/core/modules/system/lib/Drupal/system/Tests/Menu/MenuRouterTest.php
index 39494fd..7a0eb1f 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Menu/MenuRouterTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Menu/MenuRouterTest.php
@@ -60,8 +60,10 @@ function setUp() {
     $this->admin_theme = 'seven';
     $this->alternate_theme = 'stark';
     theme_enable(array($this->default_theme));
-    variable_set('theme_default', $this->default_theme);
-    config('system.theme')->set('admin', $this->admin_theme)->save();
+    config('system.theme')
+      ->set('default', $this->default_theme)
+      ->set('admin', $this->admin_theme)
+      ->save();
     theme_disable(array($this->alternate_theme));
     $this->drupalPlaceBlock('system_menu_block:menu-tools');
   }
diff --git a/core/modules/system/lib/Drupal/system/Tests/System/ThemeTest.php b/core/modules/system/lib/Drupal/system/Tests/System/ThemeTest.php
index 8cf14d5..f636849 100644
--- a/core/modules/system/lib/Drupal/system/Tests/System/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/System/ThemeTest.php
@@ -205,7 +205,9 @@ function testAdministrationTheme() {
     $this->assertRaw('core/themes/stark', 'Site default theme used on the add content page.');
 
     // Reset to the default theme settings.
-    variable_set('theme_default', 'bartik');
+    config('system.theme')
+      ->set('default', 'bartik')
+      ->save();
     $edit = array(
       'admin_theme' => '0',
       'node_admin_theme' => FALSE,
@@ -227,7 +229,7 @@ function testSwitchDefaultTheme() {
     theme_enable(array('bartik'));
     $this->drupalGet('admin/appearance');
     $this->clickLink(t('Set default'));
-    $this->assertEqual(variable_get('theme_default', ''), 'bartik');
+    $this->assertEqual(config('system.theme')->get('default'), 'bartik');
 
     // Test the default theme on the secondary links (blocks admin page).
     $this->drupalGet('admin/structure/block');
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/EntityFilteringThemeTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/EntityFilteringThemeTest.php
index 27f3e6a..3545553 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/EntityFilteringThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/EntityFilteringThemeTest.php
@@ -130,7 +130,9 @@ function testThemedEntity() {
 
     // Check each path in all available themes.
     foreach ($this->themes as $theme) {
-      variable_set('theme_default', $theme);
+      config('system.theme')
+        ->set('default', $theme)
+        ->save();
       foreach ($paths as $path) {
         $this->drupalGet($path);
         $this->assertResponse(200);
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeInfoStylesTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeInfoStylesTest.php
index c2b6c43..54ee1d2 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeInfoStylesTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeInfoStylesTest.php
@@ -34,7 +34,9 @@ public static function getInfo() {
    */
   function testStylesheets() {
     theme_enable(array('test_basetheme', 'test_subtheme'));
-    variable_set('theme_default', 'test_subtheme');
+    config('system.theme')
+      ->set('default', 'test_subtheme')
+      ->save();
 
     $base = drupal_get_path('theme', 'test_basetheme');
     // Unlike test_basetheme (and the original module CSS), the subtheme decides
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 6e9b383..ce5f96a 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -132,7 +132,9 @@ function testCSSOverride() {
    * Ensures a themes template is overrideable based on the 'template' filename.
    */
   function testTemplateOverride() {
-    variable_set('theme_default', 'test_theme');
+    config('system.theme')
+      ->set('default', 'test_theme')
+      ->save();
     $this->drupalGet('theme-test/template-test');
     $this->assertText('Success: Template overridden.', 'Template overridden by defined \'template\' filename.');
   }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
index 13b649c..c62f7c3 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
@@ -38,7 +38,9 @@ function setUp() {
    * Ensures a themes template is overrideable based on the 'template' filename.
    */
   function testTemplateOverride() {
-    variable_set('theme_default', 'test_theme_twig');
+    config('system.theme')
+      ->set('default', 'test_theme_twig')
+      ->save();
     $this->drupalGet('theme-test/template-test');
     $this->assertText('Success: Template overridden.', t('Template overridden by defined \'template\' filename.'));
   }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
index 401e021..0bcd1a6 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
@@ -35,7 +35,7 @@ public static function getInfo() {
   function testTwigDebugMarkup() {
     $extension = twig_extension();
     theme_enable(array('test_theme_twig'));
-    variable_set('theme_default', 'test_theme_twig');
+    config('system.theme')->set('default', 'test_theme_twig')->save();
     // Enable debug, rebuild the service container, and clear all caches.
     $this->settingsSet('twig_debug', TRUE);
     $this->rebuildContainer();
diff --git a/core/modules/system/lib/Drupal/system/Tests/Upgrade/BlockUpgradePathTest.php b/core/modules/system/lib/Drupal/system/Tests/Upgrade/BlockUpgradePathTest.php
index 36033bd..b9129e3 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Upgrade/BlockUpgradePathTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Upgrade/BlockUpgradePathTest.php
@@ -46,7 +46,7 @@ public function testBlockUpgradeTitleLength() {
       'machine_name' => strtolower($this->randomName(8)),
       'region' => 'sidebar_first',
     );
-    $this->drupalPost('admin/structure/block/add/system_powered_by_block/' . variable_get('theme_default', 'stark'), $settings, t('Save block'));
+    $this->drupalPost('admin/structure/block/add/system_powered_by_block/' . config('system.theme')->get('default'), $settings, t('Save block'));
     $this->assertText($settings['label'], 'Block with title longer than 64 characters successfully created.');
 
     // Try to add a block with a title over 255 characters.
@@ -55,7 +55,7 @@ public function testBlockUpgradeTitleLength() {
       'machine_name' => strtolower($this->randomName(8)),
       'region' => 'sidebar_first',
     );
-    $this->drupalPost('admin/structure/block/add/system_powered_by_block/' . variable_get('theme_default', 'stark'), $settings, t('Save block'));
+    $this->drupalPost('admin/structure/block/add/system_powered_by_block/' . config('system.theme')->get('default'), $settings, t('Save block'));
 
     // Confirm that the custom block cannot be created with title longer than
     // the maximum number of characters.
diff --git a/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
index a2c7011..b367033 100644
--- a/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -124,7 +124,7 @@ function system_themes_page() {
   $themes = system_rebuild_theme_data();
   uasort($themes, 'system_sort_modules_by_info_name');
 
-  $theme_default = variable_get('theme_default', 'stark');
+  $theme_default = config('system.theme')->get('default');
   $theme_groups  = array();
   $admin_theme = config('system.theme')->get('admin');
 
@@ -315,7 +315,7 @@ function system_theme_disable() {
     // Check if the specified theme is one recognized by the system.
     if (!empty($themes[$theme])) {
       // Do not disable the default or admin theme.
-      if ($theme == variable_get('theme_default', 'stark') || $theme == config('system.theme')->get('admin')) {
+      if ($theme === config('system.theme')->get('default') || $theme === config('system.theme')->get('admin')) {
         drupal_set_message(t('%theme is the default theme and cannot be disabled.', array('%theme' => $themes[$theme]->info['name'])), 'error');
       }
       else {
@@ -347,7 +347,9 @@ function system_theme_default() {
        theme_enable(array($theme));
       }
       // Set the default theme.
-      variable_set('theme_default', $theme);
+      config('system.theme')
+        ->set('default', $theme)
+        ->save();
 
       // Rebuild the menu. This duplicates the menu_router_rebuild() in
       // theme_enable(). However, modules must know the current default theme in
diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index daac6ba..c21d62a 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -521,10 +521,12 @@ function system_requirements($phase) {
  * Implements hook_install().
  */
 function system_install() {
-  // Enable the default theme. Can't use theme_enable() this early in
+  // Enable and set the default theme. Can't use theme_enable() this early in
   // installation.
-  variable_set('theme_default', 'stark');
   config_install_default_config('theme', 'stark');
+  config('system.theme')
+    ->set('default', 'stark')
+    ->save();
 
   // Populate the cron key state variable.
   $cron_key = drupal_hash_base64(drupal_random_bytes(55));
@@ -1972,7 +1974,8 @@ function system_update_8040() {
  */
 function system_update_8041() {
   update_variables_to_config('system.theme', array(
-    'admin_theme' => 'admin'
+    'admin_theme' => 'admin',
+    'theme_default' => 'default'
   ));
 }
 
diff --git a/core/modules/system/tests/modules/design_test/design_test.module b/core/modules/system/tests/modules/design_test/design_test.module
index 9771ba4..a7ab1b5 100644
--- a/core/modules/system/tests/modules/design_test/design_test.module
+++ b/core/modules/system/tests/modules/design_test/design_test.module
@@ -98,7 +98,7 @@ function design_test_menu_local_tasks_alter(&$data, $router_item, $root_path) {
     $actions = &$data['actions']['output'];
     // Determine the currently selected theme, if any.
     $selected_theme = drupal_container()->get('request')->query->get('theme');
-    $default_theme = variable_get('theme_default', 'stark');
+    $default_theme = config('system.theme')->get('default');
 
     // Expand all enabled themes into action links.
     $themes = list_themes();
diff --git a/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php b/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php
index 35a65ea..c14a562 100644
--- a/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php
+++ b/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php
@@ -25,7 +25,9 @@ function setUp() {
 
     // Make sure we are using distinct default and administrative themes for
     // the duration of these tests.
-    variable_set('theme_default', 'bartik');
+    config('system.theme')
+      ->set('default', 'bartik')
+      ->save();
     theme_enable(array('seven'));
     config('system.theme')->set('admin', 'seven')->save();
 
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php
index 584ea9b..b67b061 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php
@@ -1715,7 +1715,7 @@ public function buildOptionsForm(&$form, &$form_state) {
           $this->theme = $theme;
         }
         elseif (empty($this->theme)) {
-          $this->theme = variable_get('theme_default', 'bartik');
+          $this->theme = config('system.theme')->get('default');
         }
 
         if (isset($GLOBALS['theme']) && $GLOBALS['theme'] == $this->theme) {
diff --git a/core/modules/views/lib/Drupal/views/Tests/UI/OverrideDisplaysTest.php b/core/modules/views/lib/Drupal/views/Tests/UI/OverrideDisplaysTest.php
index b60f9d4..b4fc97a 100644
--- a/core/modules/views/lib/Drupal/views/Tests/UI/OverrideDisplaysTest.php
+++ b/core/modules/views/lib/Drupal/views/Tests/UI/OverrideDisplaysTest.php
@@ -54,7 +54,7 @@ function testOverrideDisplays() {
     $this->assertText($original_title);
 
     // Confirm that the view block is available in the block administration UI.
-    $this->drupalGet('admin/structure/block/list/block_plugin_ui:' . variable_get('theme_default', 'stark') . '/add');
+    $this->drupalGet('admin/structure/block/list/block_plugin_ui:' . config('system.theme')->get('default') . '/add');
     $this->assertText('View: ' . $view['human_name']);
 
     // Place the block.
@@ -113,7 +113,7 @@ function testWizardMixedDefaultOverriddenDisplays() {
     $this->assertNoText($view['block[title]']);
 
     // Confirm that the block is available in the block administration UI.
-    $this->drupalGet('admin/structure/block/list/block_plugin_ui:' . variable_get('theme_default', 'stark') . '/add');
+    $this->drupalGet('admin/structure/block/list/block_plugin_ui:' . config('system.theme')->get('default') . '/add');
     $this->assertText('View: ' . $view['human_name']);
 
     // Place the block.
diff --git a/core/modules/views/lib/Drupal/views/Tests/Wizard/BasicTest.php b/core/modules/views/lib/Drupal/views/Tests/Wizard/BasicTest.php
index b786859..8e15322 100644
--- a/core/modules/views/lib/Drupal/views/Tests/Wizard/BasicTest.php
+++ b/core/modules/views/lib/Drupal/views/Tests/Wizard/BasicTest.php
@@ -123,7 +123,7 @@ function testViewsWizardAndListing() {
     $this->assertLinkByHref(url($view3['page[path]']));
 
     // Confirm that the block is available in the block administration UI.
-    $this->drupalGet('admin/structure/block/list/block_plugin_ui:' . variable_get('theme_default', 'stark') . '/add');
+    $this->drupalGet('admin/structure/block/list/block_plugin_ui:' . config('system.theme')->get('default') . '/add');
     $this->assertText('View: ' . $view3['human_name']);
 
     // Place the block.
diff --git a/core/modules/views/lib/Drupal/views/Tests/Wizard/ItemsPerPageTest.php b/core/modules/views/lib/Drupal/views/Tests/Wizard/ItemsPerPageTest.php
index 16e86fa..51ba883 100644
--- a/core/modules/views/lib/Drupal/views/Tests/Wizard/ItemsPerPageTest.php
+++ b/core/modules/views/lib/Drupal/views/Tests/Wizard/ItemsPerPageTest.php
@@ -75,7 +75,7 @@ function testItemsPerPage() {
     $this->assertTrue($pos5 < $pos4 && $pos4 < $pos3 && $pos3 < $pos2, t('The nodes appear in the expected order in the page display.'));
 
     // Confirm that the block is listed in the block administration UI.
-    $this->drupalGet('admin/structure/block/list/block_plugin_ui:' . variable_get('theme_default', 'stark') . '/add');
+    $this->drupalGet('admin/structure/block/list/block_plugin_ui:' . config('system.theme')->get('default') . '/add');
     $this->assertText('View: ' . $view['human_name']);
     // Place the block, visit a page that displays the block, and check that the
     // nodes we expect appear in the correct order.
diff --git a/core/profiles/standard/standard.install b/core/profiles/standard/standard.install
index 85c818c..13463b9 100644
--- a/core/profiles/standard/standard.install
+++ b/core/profiles/standard/standard.install
@@ -15,7 +15,9 @@ function standard_install() {
   // Enable Bartik theme and set it as default theme instead of Stark.
   // @see system_install()
   $default_theme = 'bartik';
-  variable_set('theme_default', $default_theme);
+  config('system.theme')
+    ->set('default', $default_theme)
+    ->save();
   theme_enable(array($default_theme));
   theme_disable(array('stark'));
 
diff --git a/sites/default/default.settings.php b/sites/default/default.settings.php
index 4c1239a..b2fbc90 100644
--- a/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -531,12 +531,12 @@
  *
  * The following overrides are examples:
  * - site_name: Defines the site's name.
- * - theme_default: Defines the default theme for this site.
+ * - $conf['system.theme']['default']: Defines the default theme for this site.
  * - anonymous: Defines the human-readable name of anonymous users.
  * Remove the leading hash signs to enable.
  */
 # $conf['system.site']['name'] = 'My Drupal site';
-# $conf['theme_default'] = 'stark';
+# $conf['system.theme']['default'] = 'stark';
 # $conf['anonymous'] = 'Visitor';
 
 /**