reverted:
--- b/core/includes/ajax.inc
+++ a/core/includes/ajax.inc
@@ -411,7 +411,7 @@
     // 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 $theme;
     }
   }
diff -u b/core/includes/theme.inc b/core/includes/theme.inc
--- b/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -63,7 +63,7 @@
  * @see drupal_theme_access()
  */
 function _drupal_theme_access($theme) {
-  $admin_theme = config('system.theme')->get('admin');
+  $admin_theme = variable_get('admin_theme');
   return !empty($theme->status) || ($admin_theme && $theme->name == $admin_theme);
 }
 
@@ -83,7 +83,7 @@
 
   // 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 : config('system.theme')->get('default');
+  $theme = !empty($user->theme) && drupal_theme_access($user->theme) ? $user->theme : variable_get('theme_default', 'stark');
 
   // 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.
@@ -1288,7 +1288,6 @@
  * @return
  *   The value of the requested setting, NULL if the setting does not exist.
  */
-
 function theme_get_setting($setting_name, $theme = NULL) {
   $cache = &drupal_static(__FUNCTION__, array());
 
@@ -1350,7 +1349,7 @@
     $cache[$theme]->setData(NestedArray::mergeDeep($cache[$theme]->get(), config('system.theme.global')->get()));
 
     if ($theme) {
-      // Get the saved theme-specific settings configuration.
+      // Get the saved theme-specific settings from the configuration system.
       $cache[$theme]->setData(NestedArray::mergeDeep($cache[$theme]->get(), config($theme . '.settings')->get()));
 
       // If the theme does not support a particular feature, override the global
@@ -1495,7 +1494,7 @@
  */
 function theme_disable($theme_list) {
   // Don't disable the default theme.
-  if ($pos = array_search(config('system.theme')->get('default'), $theme_list) !== FALSE) {
+  if ($pos = array_search(variable_get('theme_default', 'stark'), $theme_list) !== FALSE) {
     unset($theme_list[$pos]);
     if (empty($theme_list)) {
       return;
reverted:
--- b/core/includes/theme.maintenance.inc
+++ a/core/includes/theme.maintenance.inc
@@ -50,11 +50,7 @@
     //   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'));
-    $config = config('system.theme');
-    $custom_theme = $config->get('maintenance');
-    if (empty($custom_theme)) {
-      $custom_theme = $config->get('default');
-    }
   }
 
   // Ensure that system.module is loaded.
reverted:
--- b/core/modules/block/block.admin.inc
+++ a/core/modules/block/block.admin.inc
@@ -307,9 +307,8 @@
     '#tree' => TRUE,
   );
 
+  $theme_default = variable_get('theme_default', 'stark');
+  $admin_theme = variable_get('admin_theme');
-  $config = config('system.theme');
-  $theme_default = $config->get('default');
-  $admin_theme = $config->get('admin');
   foreach (list_themes() as $key => $theme) {
     // Only display enabled themes
     if ($theme->status) {
reverted:
--- b/core/modules/block/block.module
+++ a/core/modules/block/block.module
@@ -63,7 +63,7 @@
       return '<p>' . t('Use this page to create a new custom block.') . '</p>';
   }
   if ($arg[0] == 'admin' && $arg[1] == 'structure' && $arg['2'] == 'block' && (empty($arg[3]) || $arg[3] == 'list')) {
+    $demo_theme = !empty($arg[4]) ? $arg[4] : variable_get('theme_default', 'stark');
-    $demo_theme = !empty($arg[4]) ? $arg[4] : 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>';
     $output .= '<p>' . l(t('Demonstrate block regions (@theme)', array('@theme' => $themes[$demo_theme]->info['name'])), 'admin/structure/block/demo/' . $demo_theme) . '</p>';
@@ -103,7 +103,7 @@
  * Implements hook_menu().
  */
 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.',
@@ -306,7 +306,7 @@
       $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'))),
@@ -674,7 +674,7 @@
   // Initialize theme's blocks if none already registered.
   $has_blocks = (bool) db_query_range('SELECT 1 FROM {block} WHERE theme = :theme', 0, 1, array(':theme' => $theme))->fetchField();
   if (!$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);
     $result = db_query("SELECT * FROM {block} WHERE theme = :theme", array(':theme' => $default_theme), array('fetch' => PDO::FETCH_ASSOC));
reverted:
--- b/core/modules/block/lib/Drupal/block/Tests/BlockHiddenRegionTest.php
+++ a/core/modules/block/lib/Drupal/block/Tests/BlockHiddenRegionTest.php
@@ -29,7 +29,7 @@
       ->key(array(
         'module' => 'search',
         'delta' => 'form',
+        'theme' => variable_get('theme_default', 'stark'),
-        'theme' => config('system.theme')->get('default'),
       ))
       ->fields(array(
         'status' => 1,
@@ -56,7 +56,7 @@
     // 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.
reverted:
--- b/core/modules/block/lib/Drupal/block/Tests/BlockInvalidRegionTest.php
+++ a/core/modules/block/lib/Drupal/block/Tests/BlockInvalidRegionTest.php
@@ -32,13 +32,12 @@
    * Tests that blocks assigned to invalid regions work correctly.
    */
   function testBlockInInvalidRegion() {
-    $config = config('system.theme');
     // Enable a test block in the default theme and place it in an invalid region.
     db_merge('block')
       ->key(array(
         'module' => 'block_test',
         'delta' => 'test_html_id',
+        'theme' => variable_get('theme_default', 'stark'),
-        'theme' => $config->get('default'),
       ))
       ->fields(array(
         'status' => 1,
@@ -62,7 +61,7 @@
       ->key(array(
         'module' => 'block_test',
         'delta' => 'test_html_id',
+        'theme' => variable_get('theme_default', 'stark'),
-        'theme' => $config->get('default'),
       ))
       ->fields(array(
         'region' => 'invalid_region',
reverted:
--- b/core/modules/block/lib/Drupal/block/Tests/NewDefaultThemeBlocksTest.php
+++ a/core/modules/block/lib/Drupal/block/Tests/NewDefaultThemeBlocksTest.php
@@ -35,9 +35,8 @@
 
     // Ensure no other theme's blocks are in the block table yet.
     $themes = array();
+    $themes['default'] = variable_get('theme_default', 'stark');
+    if ($admin_theme = variable_get('admin_theme')) {
-    $config = config('system.theme');
-    $themes['default'] = $config->get('default');
-    if ($admin_theme = $config->get('admin')) {
       $themes['admin'] = $admin_theme;
     }
     $count = db_query_range('SELECT 1 FROM {block} WHERE theme NOT IN (:themes)', 0, 1, array(':themes' => $themes))->fetchField();
@@ -56,7 +55,7 @@
     // the default theme had.
     $new_theme = 'bartik';
     theme_enable(array($new_theme));
+    variable_set('theme_default', $new_theme);
-    config('system.theme')->set('default', $new_theme)->save();
     $result = db_query('SELECT * FROM {block} WHERE theme = :theme', array(':theme' => $new_theme));
     foreach ($result as $block) {
       unset($block->theme, $block->bid);
reverted:
--- b/core/modules/color/lib/Drupal/color/Tests/ColorTest.php
+++ a/core/modules/color/lib/Drupal/color/Tests/ColorTest.php
@@ -68,7 +68,7 @@
    * Tests the Color module functionality using the given theme.
    */
   function _testColor($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);
@@ -114,7 +114,7 @@
    * 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);
reverted:
--- b/core/modules/comment/lib/Drupal/comment/Tests/CommentInterfaceTest.php
+++ a/core/modules/comment/lib/Drupal/comment/Tests/CommentInterfaceTest.php
@@ -430,7 +430,7 @@
   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.
reverted:
--- b/core/modules/node/lib/Drupal/node/Tests/NodeBlockFunctionalTest.php
+++ a/core/modules/node/lib/Drupal/node/Tests/NodeBlockFunctionalTest.php
@@ -115,14 +115,13 @@
     $this->assertText($node4->label(), t('Node found in block.'));
 
     // Create the custom block.
-    $theme_config = config('system.theme');
     $custom_block = array();
     $custom_block['info'] = $this->randomName();
     $custom_block['title'] = $this->randomName();
     $custom_block['types[article]'] = TRUE;
     $custom_block['body[value]'] = $this->randomName(32);
+    $custom_block['regions[' . variable_get('theme_default', 'stark') . ']'] = 'content';
+    if ($admin_theme = variable_get('admin_theme')) {
-    $custom_block['regions[' . $theme_config->get('default') . ']'] = 'content';
-    if ($admin_theme = $theme_config->get('admin')) {
       $custom_block['regions[' . $admin_theme . ']'] = 'content';
     }
     $this->drupalPost('admin/structure/block/add', $custom_block, t('Save block'));
reverted:
--- b/core/modules/openid/lib/Drupal/openid/Tests/OpenIDTestBase.php
+++ a/core/modules/openid/lib/Drupal/openid/Tests/OpenIDTestBase.php
@@ -27,7 +27,7 @@
       ->key(array(
         'module' => 'user',
         'delta' => 'login',
+        'theme' => variable_get('theme_default', 'stark'),
-        'theme' => config('system.theme')->get('default'),
       ))
       ->fields(array(
         'status' => 1,
reverted:
--- b/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
+++ a/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
@@ -62,7 +62,7 @@
    */
   function testShortcutQuickLink() {
     theme_enable(array('seven'));
+    variable_set('admin_theme', 'seven');
-    config('system.theme')->set('admin', 'seven')->save();
     variable_set('node_admin_theme', TRUE);
     $this->drupalGet($this->set->links[0]['link_path']);
     $this->assertRaw(t('Remove from %title shortcuts', array('%title' => $this->set->title)), '"Add to shortcuts" link properly switched to "Remove from shortcuts".');
@@ -120,7 +120,7 @@
    */
   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', t('Add to shortcuts link was not shown on a page not found.'));
reverted:
--- b/core/modules/statistics/lib/Drupal/statistics/Tests/StatisticsReportsTest.php
+++ a/core/modules/statistics/lib/Drupal/statistics/Tests/StatisticsReportsTest.php
@@ -83,7 +83,7 @@
 
     // Configure and save the block.
     $block = block_load('statistics', 'popular');
+    $block->theme = variable_get('theme_default', 'stark');
-    $block->theme = config('system.theme')->get('default');
     $block->status = 1;
     $block->pages = '';
     $block->region = 'sidebar_first';
reverted:
--- b/core/modules/system/config/system.theme.yml
+++ /dev/null
@@ -1,3 +0,0 @@
-admin: '0'
-default: stark
-maintenance: ''
reverted:
--- b/core/modules/system/lib/Drupal/system/Tests/Ajax/FrameworkTest.php
+++ a/core/modules/system/lib/Drupal/system/Tests/Ajax/FrameworkTest.php
@@ -154,7 +154,7 @@
     // 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.
reverted:
--- b/core/modules/system/lib/Drupal/system/Tests/Batch/PageTest.php
+++ a/core/modules/system/lib/Drupal/system/Tests/Batch/PageTest.php
@@ -31,10 +31,8 @@
   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');
+    variable_set('admin_theme', 'seven');
-    config('system.theme')
-      ->set('default', 'bartik')
-      ->set('admin', 'seven')
-      ->save();
     // Log in as an administrator who can see the administrative theme.
     $admin_user = $this->drupalCreateUser(array('view the administration theme'));
     $this->drupalLogin($admin_user);
reverted:
--- b/core/modules/system/lib/Drupal/system/Tests/Menu/RouterTest.php
+++ a/core/modules/system/lib/Drupal/system/Tests/Menu/RouterTest.php
@@ -29,10 +29,8 @@
     // Make the tests below more robust by explicitly setting the default theme
     // and administrative theme that they expect.
     theme_enable(array('bartik'));
+    variable_set('theme_default', 'bartik');
+    variable_set('admin_theme', 'seven');
-    config('system.theme')
-      ->set('default', 'bartik')
-      ->set('admin', 'seven')
-      ->save();
     theme_disable(array('stark'));
 
     // Enable navigation menu block.
reverted:
--- b/core/modules/system/lib/Drupal/system/Tests/System/ThemeTest.php
+++ a/core/modules/system/lib/Drupal/system/Tests/System/ThemeTest.php
@@ -165,8 +165,7 @@
    */
   function testAdministrationTheme() {
     theme_enable(array('stark'));
+    variable_set('theme_default', 'stark');
-    $config = config('system.theme');
-    $config->set('default', 'stark')->save();
     // Enable an administration theme and show it on the node admin pages.
     $edit = array(
       'admin_theme' => 'seven',
@@ -199,7 +198,7 @@
     $this->assertRaw('core/themes/stark', t('Site default theme used on the add content page.'));
 
     // Reset to the default theme settings.
+    variable_set('theme_default', 'bartik');
-    $config->set('default', 'bartik')->save();
     $edit = array(
       'admin_theme' => '0',
       'node_admin_theme' => FALSE,
@@ -221,7 +220,7 @@
     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');
reverted:
--- b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -124,7 +124,7 @@
    * 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.', t('Template overridden by defined \'template\' filename.'));
   }
diff -u b/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
--- b/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -118,7 +118,7 @@
   $themes = system_rebuild_theme_data();
   uasort($themes, 'system_sort_modules_by_info_name');
 
-  $theme_default = config('system.theme')->get('default');
+  $theme_default = variable_get('theme_default', 'stark');
   $theme_groups  = array();
 
   foreach ($themes as &$theme) {
@@ -241,13 +241,12 @@
     '#type' => 'fieldset',
     '#title' => t('Administration theme'),
   );
-  $admin_theme = config('system.theme')->get('admin');
   $form['admin_theme']['admin_theme'] = array(
     '#type' => 'select',
     '#options' => array(0 => t('Default theme')) + $theme_options,
     '#title' => t('Administration theme'),
     '#description' => t('Choose "Default theme" to always use the same theme as the rest of the site.'),
-    '#default_value' => empty($admin_theme) ? 0 : $admin_theme,
+    '#default_value' => variable_get('admin_theme', 0),
   );
   $form['admin_theme']['actions'] = array('#type' => 'actions');
   $form['admin_theme']['actions']['submit'] = array(
@@ -262,7 +261,7 @@
  */
 function system_themes_admin_form_submit($form, &$form_state) {
   drupal_set_message(t('The configuration options have been saved.'));
-  config('system.theme')->set('admin', $form_state['values']['admin_theme'])->save();
+  variable_set('admin_theme', $form_state['values']['admin_theme']);
 }
 
 /**
@@ -298,7 +297,7 @@
 
     // Check if the specified theme is one recognized by the system.
     if (!empty($themes[$theme])) {
-      if ($theme == config('system.theme')->get('default')) {
+      if ($theme == variable_get('theme_default', 'stark')) {
         // Don't disable the default theme.
         drupal_set_message(t('%theme is the default theme and cannot be disabled.', array('%theme' => $themes[$theme]->info['name'])), 'error');
       }
@@ -331,8 +330,7 @@
        theme_enable(array($theme));
       }
       // Set the default theme.
-      $config = config('system.theme');
-      $config->set('default', $theme)->save();
+      variable_set('theme_default', $theme);
 
       // Rebuild the menu. This duplicates the menu_router_rebuild() in
       // theme_enable(). However, modules must know the current default theme in
@@ -344,7 +342,7 @@
 
       // The status message depends on whether an admin theme is currently in use:
       // a value of 0 means the admin theme is set to be the default theme.
-      $admin_theme = $config->get('admin');
+      $admin_theme = variable_get('admin_theme', 0);
       if ($admin_theme != 0 && $admin_theme != $theme) {
         drupal_set_message(t('Please note that the administration theme is still set to the %admin_theme theme; consequently, the theme on this page remains unchanged. All non-administrative sections of the site, however, will show the selected %selected_theme theme by default.', array(
           '%admin_theme' => $themes[$admin_theme]->info['name'],
diff -u b/core/modules/system/system.install b/core/modules/system/system.install
--- b/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -507,7 +507,7 @@
  */
 function system_install() {
   // Enable the default theme.
-  config('system.theme')->set('default', 'stark')->save();
+  variable_set('theme_default', 'stark');
   db_update('system')
     ->fields(array('status' => 1))
     ->condition('type', 'theme')
@@ -2019,12 +2019,6 @@
     }
     $new_config->save();
   }
-
-  update_variables_to_config('system.theme', array(
-    'admin_theme' => 'admin',
-    'theme_default' => 'default',
-    'maintenance_theme' => 'maintenance',
-  ));
 }
 
 /**
reverted:
--- b/core/modules/system/system.module
+++ a/core/modules/system/system.module
@@ -237,7 +237,7 @@
     ),
     'view the administration theme' => array(
       'title' => t('View the administration theme'),
+      'description' => variable_get('admin_theme') ? '' : t('This is only used when the site is configured to use a separate administration theme on the <a href="@appearance-url">Appearance</a> page.', array('@appearance-url' => url('admin/appearance'))),
-      'description' => config('system.theme')->get('admin') ? '' : t('This is only used when the site is configured to use a separate administration theme on the <a href="@appearance-url">Appearance</a> page.', array('@appearance-url' => url('admin/appearance'))),
     ),
     'access site reports' => array(
       'title' => t('View site reports'),
@@ -2202,7 +2202,7 @@
   if ($request = request()) {
     $path = $request->attributes->get('system_path');
     if (user_access('view the administration theme') && path_is_admin($path)) {
+      return variable_get('admin_theme');
-      return config('system.theme')->get('admin');
     }
   }
 }
reverted:
--- b/core/modules/system/tests/modules/menu_test/menu_test.module
+++ a/core/modules/system/tests/modules/menu_test/menu_test.module
@@ -462,7 +462,7 @@
 function menu_test_theme_callback($argument) {
   // Test using the variable administrative theme.
   if ($argument == 'use-admin-theme') {
+    return variable_get('admin_theme');
-    return config('system.theme')->get('admin');
   }
   // Test using a theme that exists, but may or may not be enabled.
   elseif ($argument == 'use-stark-theme') {
reverted:
--- b/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php
+++ a/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php
@@ -25,10 +25,8 @@
 
     // Make sure we are using distinct default and administrative themes for
     // the duration of these tests.
+    variable_set('theme_default', 'bartik');
+    variable_set('admin_theme', 'seven');
-    config('system.theme')
-      ->set('default', 'bartik')
-      ->set('admin', 'seven')
-      ->save();
 
     // Create and log in as a user who has permission to add and edit taxonomy
     // terms and view the administrative theme.
reverted:
--- b/core/modules/user/lib/Drupal/user/Tests/UserBlocksTests.php
+++ a/core/modules/user/lib/Drupal/user/Tests/UserBlocksTests.php
@@ -29,7 +29,7 @@
       ->key(array(
         'module' => 'user',
         'delta' => 'login',
+        'theme' => variable_get('theme_default', 'stark'),
-        'theme' => config('system.theme')->get('default'),
       ))
       ->fields(array(
         'status' => 1,
reverted:
--- b/profiles/minimal/minimal.install
+++ a/profiles/minimal/minimal.install
@@ -15,7 +15,7 @@
   // 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', $default_theme);
   theme_enable(array($default_theme));
   theme_disable(array('stark'));
 
reverted:
--- b/profiles/standard/standard.install
+++ a/profiles/standard/standard.install
@@ -71,8 +71,7 @@
   // Enable Bartik theme and set it as default theme instead of Stark.
   // @see system_install()
   $default_theme = 'bartik';
+  variable_set('theme_default', $default_theme);
-  $theme_config = config('system.theme');
-  $theme_config->set('default', $default_theme)->save();
   theme_enable(array($default_theme));
   theme_disable(array('stark'));
 
@@ -410,6 +409,6 @@
     ->condition('type', 'theme')
     ->condition('name', 'seven')
     ->execute();
+  variable_set('admin_theme', 'seven');
-  $theme_config->set('admin', 'seven')->save();
   variable_set('node_admin_theme', '1');
 }