diff -u b/core/modules/system/lib/Drupal/system/Controller/SystemController.php b/core/modules/system/lib/Drupal/system/Controller/SystemController.php
--- b/core/modules/system/lib/Drupal/system/Controller/SystemController.php
+++ b/core/modules/system/lib/Drupal/system/Controller/SystemController.php
@@ -7,10 +7,12 @@
 
 namespace Drupal\system\Controller;
 
-use Drupal\Core\Access\CsrfTokenGenerator;
 use Drupal\Core\Controller\ControllerBase;
 use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
 use Drupal\Core\Entity\Query\QueryFactory;
+use Drupal\Core\Theme\ThemeAccessCheck;
+use Drupal\system\Form\ThemeAdminForm;
+use Drupal\Core\Access\CsrfTokenGenerator;
 use Drupal\system\SystemManager;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 use Symfony\Component\HttpFoundation\RedirectResponse;
@@ -42,6 +44,13 @@
   protected $tokenGenerator;
 
   /**
+   * The theme access checker service.
+   *
+   * @var \Drupal\Core\Theme\ThemeAccessCheck
+   */
+  protected $themeAccess;
+
+  /**
    * Constructs a new SystemController.
    *
    * @param \Drupal\system\SystemManager $systemManager
@@ -50,11 +59,14 @@
    *   The entity query object.
    * @param \Drupal\Core\Access\CsrfTokenGenerator $token_generator
    *   The token generator service.
+   * @param \Drupal\Core\Theme\ThemeAccessCheck $theme_access
+   *   The theme access checker service.
    */
-  public function __construct(SystemManager $systemManager, QueryFactory $queryFactory, CsrfTokenGenerator $token_generator) {
+  public function __construct(SystemManager $systemManager, QueryFactory $queryFactory, CsrfTokenGenerator $token_generator, ThemeAccessCheck $theme_access) {
     $this->systemManager = $systemManager;
     $this->queryFactory = $queryFactory;
     $this->tokenGenerator = $token_generator;
+    $this->themeAccess = $theme_access;
   }
 
   /**
@@ -64,7 +76,8 @@
     return new static(
       $container->get('system.manager'),
       $container->get('entity.query'),
-      $container->get('csrf_token')
+      $container->get('csrf_token'),
+      $container->get('access_check.theme')
     );
   }
 
@@ -218,7 +231,7 @@
       if (!empty($theme->status) || !$theme->incompatible_core && !$theme->incompatible_php && !$theme->incompatible_base && !$theme->incompatible_engine) {
         // Create the operations links.
         $query['theme'] = $theme->name;
-        if (drupal_theme_access($theme)) {
+        if ($this->themeAccess->checkAccess($theme->name)) {
           $theme->operations[] = array(
             'title' => $this->t('Settings'),
             'href' => 'admin/appearance/settings/' . $theme->name,
@@ -293,7 +306,7 @@
       '#theme_groups' => $theme_groups,
       '#theme_group_titles' => $theme_group_titles,
     );
-    $build[] = drupal_get_form('system_themes_admin_form', $admin_theme_options);
+    $build[] = drupal_get_form(new ThemeAdminForm(), $admin_theme_options);
 
     return $build;
   }
diff -u b/core/modules/system/system.module b/core/modules/system/system.module
--- b/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -2690,41 +2690,6 @@
 }
 
 /**
- * Form to select the administration theme.
- *
- * @ingroup forms
- * @see system_themes_admin_form_submit()
- */
-function system_themes_admin_form($form, &$form_state, $theme_options) {
-  // Administration theme settings.
-  $form['admin_theme'] = array(
-    '#type' => 'details',
-    '#title' => t('Administration theme'),
-  );
-  $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' => Drupal::config('system.theme')->get('admin'),
-  );
-  $form['admin_theme']['actions'] = array('#type' => 'actions');
-  $form['admin_theme']['actions']['submit'] = array(
-    '#type' => 'submit',
-    '#value' => t('Save configuration'),
-  );
-  return $form;
-}
-
-/**
- * Process system_themes_admin_form form submissions.
- */
-function system_themes_admin_form_submit($form, &$form_state) {
-  drupal_set_message(t('The configuration options have been saved.'));
-  Drupal::config('system.theme')->set('admin', $form_state['values']['admin_theme'])->save();
-}
-
-/**
  * Returns an array of default theme features.
  */
 function _system_default_theme_features() {
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/system/lib/Drupal/system/Form/ThemeAdminForm.php
@@ -0,0 +1,56 @@
+<?php
+/**
+ * @file
+ * Contains \Drupal\system\Form\ThemeAdminForm
+ */
+
+namespace Drupal\system\Form;
+
+use Drupal\Core\Form\FormBase;
+
+/**
+ * Form to select the administration theme.
+ *
+ * @ingroup forms
+ */
+class ThemeAdminForm extends FormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormID() {
+    return 'system_themes_admin_form';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, array &$form_state, array $theme_options = NULL) {
+    // Administration theme settings.
+    $form['admin_theme'] = array(
+      '#type' => 'details',
+      '#title' => t('Administration theme'),
+    );
+    $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' => $this->config('system.theme')->get('admin'),
+    );
+    $form['admin_theme']['actions'] = array('#type' => 'actions');
+    $form['admin_theme']['actions']['submit'] = array(
+      '#type' => 'submit',
+      '#value' => t('Save configuration'),
+    );
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, array &$form_state) {
+    drupal_set_message(t('The configuration options have been saved.'));
+    $this->config('system.theme')->set('admin', $form_state['values']['admin_theme'])->save();
+  }
+}