diff --git a/core/modules/block/block.admin.inc b/core/modules/block/block.admin.inc
index d21701221a36..f9fa425df61b 100644
--- a/core/modules/block/block.admin.inc
+++ b/core/modules/block/block.admin.inc
@@ -312,7 +312,7 @@ function block_admin_configure($form, &$form_state, $module, $delta) {
   );
 
   $theme_default = variable_get('theme_default', 'stark');
-  $admin_theme = config('system.site')->get('admin_theme');
+  $admin_theme = config('system.theme')->get('admin');
   foreach (list_themes() as $key => $theme) {
     // Only display enabled themes
     if ($theme->status) {
diff --git a/core/modules/block/lib/Drupal/block/Tests/NewDefaultThemeBlocksTest.php b/core/modules/block/lib/Drupal/block/Tests/NewDefaultThemeBlocksTest.php
index b8a2ef2e6046..85247b511125 100644
--- a/core/modules/block/lib/Drupal/block/Tests/NewDefaultThemeBlocksTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/NewDefaultThemeBlocksTest.php
@@ -40,7 +40,7 @@ class NewDefaultThemeBlocksTest extends WebTestBase {
     // Ensure no other theme's blocks are in the block table yet.
     $themes = array();
     $themes['default'] = variable_get('theme_default', 'stark');
-    if ($admin_theme = config('system.site')->get('admin_theme')) {
+    if ($admin_theme = config('system.theme')->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();
diff --git a/core/modules/node/lib/Drupal/node/Tests/NodeBlockFunctionalTest.php b/core/modules/node/lib/Drupal/node/Tests/NodeBlockFunctionalTest.php
index 9533a1c15e79..1c1ffb1f1b00 100644
--- a/core/modules/node/lib/Drupal/node/Tests/NodeBlockFunctionalTest.php
+++ b/core/modules/node/lib/Drupal/node/Tests/NodeBlockFunctionalTest.php
@@ -129,7 +129,7 @@ class NodeBlockFunctionalTest extends NodeTestBase {
     $custom_block['types[article]'] = TRUE;
     $custom_block['body[value]'] = $this->randomName(32);
     $custom_block['regions[' . variable_get('theme_default', 'stark') . ']'] = 'content';
-    if ($admin_theme = config('system.site')->get('admin_theme')) {
+    if ($admin_theme = config('system.theme')->get('admin')) {
       $custom_block['regions[' . $admin_theme . ']'] = 'content';
     }
     $this->drupalPost('admin/structure/block/add', $custom_block, t('Save block'));
diff --git a/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php b/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
index ad12639b3664..cd453030b782 100644
--- a/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
+++ b/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
@@ -64,7 +64,7 @@ class ShortcutLinksTest extends ShortcutTestBase {
    */
   function testShortcutQuickLink() {
     theme_enable(array('seven'));
-    config('system.site')->set('admin_theme', 'seven')->save();
+    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".');
diff --git a/core/modules/system/config/system.site.yml b/core/modules/system/config/system.site.yml
index 1776d40de1e1..b642dc83ecf2 100644
--- a/core/modules/system/config/system.site.yml
+++ b/core/modules/system/config/system.site.yml
@@ -1,4 +1,3 @@
-admin_theme: '0'
 name: Drupal
 mail: ''
 slogan: ''
diff --git a/core/modules/system/config/system.theme.yml b/core/modules/system/config/system.theme.yml
index 40fad066540b..cf330e1840da 100644
--- a/core/modules/system/config/system.theme.yml
+++ b/core/modules/system/config/system.theme.yml
@@ -1,2 +1,3 @@
+admin: '0'
 enabled:
   stark: '0'
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 9be755123023..97593f23cdb7 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Batch/PageTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Batch/PageTest.php
@@ -37,7 +37,7 @@ class PageTest extends WebTestBase {
     // is using a different theme than would normally be used by the batch API.
     variable_set('theme_default', 'bartik');
     theme_enable(array('seven'));
-    config('system.site')->set('admin_theme', 'seven')->save();
+    config('system.theme')->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);
diff --git a/core/modules/system/lib/Drupal/system/Tests/Menu/RouterTest.php b/core/modules/system/lib/Drupal/system/Tests/Menu/RouterTest.php
index 82458d33ae3e..179ea9347d8e 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Menu/RouterTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Menu/RouterTest.php
@@ -38,7 +38,7 @@ class RouterTest extends WebTestBase {
     // and administrative theme that they expect.
     theme_enable(array('bartik'));
     variable_set('theme_default', 'bartik');
-    config('system.site')->set('admin_theme', 'seven')->save();
+    config('system.theme')->set('admin', 'seven')->save();
     theme_disable(array('stark'));
 
     // Enable navigation menu block.
diff --git a/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
index 5d6a70f75c18..a5194f73b516 100644
--- a/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -120,7 +120,7 @@ function system_themes_page() {
 
   $theme_default = variable_get('theme_default', 'stark');
   $theme_groups  = array();
-  $admin_theme = config('system.site')->get('admin_theme');
+  $admin_theme = config('system.theme')->get('admin');
 
   foreach ($themes as &$theme) {
     if (!empty($theme->info['hidden'])) {
@@ -257,7 +257,7 @@ function system_themes_admin_form($form, &$form_state, $theme_options) {
     '#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' => config('system.site')->get('admin_theme'),
+    '#default_value' => config('system.theme')->get('admin'),
   );
   $form['admin_theme']['actions'] = array('#type' => 'actions');
   $form['admin_theme']['actions']['submit'] = array(
@@ -272,7 +272,7 @@ function system_themes_admin_form($form, &$form_state, $theme_options) {
  */
 function system_themes_admin_form_submit($form, &$form_state) {
   drupal_set_message(t('The configuration options have been saved.'));
-  config('system.site')->set('admin_theme', $form_state['values']['admin_theme'])->save();
+  config('system.theme')->set('admin', $form_state['values']['admin_theme'])->save();
 }
 
 /**
@@ -309,7 +309,7 @@ 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') || $theme == config('system.site')->get('admin_theme')) {
+      if ($theme == variable_get('theme_default', 'stark') || $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 {
@@ -353,7 +353,7 @@ function system_theme_default() {
 
       // 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('system.site')->get('admin_theme');
+      $admin_theme = config('system.theme')->get('admin');
       if ($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 --git a/core/modules/system/system.install b/core/modules/system/system.install
index a1ad5461b6e8..06935d16e509 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -2325,8 +2325,8 @@ function system_update_8040() {
  * @ingroup config_upgrade
  */
 function system_update_8041() {
-  update_variables_to_config('system.site', array(
-    'admin_theme' => 'admin_theme'
+  update_variables_to_config('system.theme', array(
+    'admin_theme' => 'admin'
   ));
 }
 
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index c3df4c1147bf..139fb0f45b81 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -214,7 +214,7 @@ function system_permission() {
     ),
     'view the administration theme' => array(
       'title' => t('View the administration theme'),
-      'description' => config('system.site')->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'),
@@ -2394,7 +2394,7 @@ function system_custom_theme() {
     $request = drupal_container()->get('request');
     $path = $request->attributes->get('system_path');
     if (user_access('view the administration theme') && path_is_admin($path)) {
-      return config('system.site')->get('admin_theme');
+      return config('system.theme')->get('admin');
     }
   }
 }
diff --git a/core/modules/system/tests/modules/menu_test/menu_test.module b/core/modules/system/tests/modules/menu_test/menu_test.module
index f3bd0337e59a..36cef57fa715 100644
--- a/core/modules/system/tests/modules/menu_test/menu_test.module
+++ b/core/modules/system/tests/modules/menu_test/menu_test.module
@@ -462,7 +462,7 @@ function menu_test_theme_page_callback($inherited = FALSE) {
 function menu_test_theme_callback($argument) {
   // Test using the variable administrative theme.
   if ($argument == 'use-admin-theme') {
-    return config('system.site')->get('admin_theme');
+    return config('system.theme')->get('admin');
   }
   // Test using a theme that exists, but may or may not be enabled.
   elseif ($argument == 'use-stark-theme') {
diff --git a/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php b/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php
index e555bb6c77a0..8abad54e8406 100644
--- a/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php
+++ b/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php
@@ -27,7 +27,7 @@ class ThemeTest extends TaxonomyTestBase {
     // the duration of these tests.
     variable_set('theme_default', 'bartik');
     theme_enable(array('seven'));
-    config('system.site')->set('admin_theme', 'seven')->save();
+    config('system.theme')->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.
diff --git a/core/profiles/standard/standard.install b/core/profiles/standard/standard.install
index e2d65f053086..b24946ef2c49 100644
--- a/core/profiles/standard/standard.install
+++ b/core/profiles/standard/standard.install
@@ -437,6 +437,6 @@ function standard_install() {
 
   // Enable the admin theme.
   theme_enable(array('seven'));
-  config('system.site')->set('admin_theme', 'seven')->save();
+  config('system.theme')->set('admin', 'seven')->save();
   variable_set('node_admin_theme', '1');
 }