diff --git a/core/modules/system/lib/Drupal/system/Controller/SystemController.php b/core/modules/system/lib/Drupal/system/Controller/SystemController.php
index 28861ad..6ec58fe 100644
--- a/core/modules/system/lib/Drupal/system/Controller/SystemController.php
+++ b/core/modules/system/lib/Drupal/system/Controller/SystemController.php
@@ -322,14 +322,6 @@ public function themesPage() {
   }

   /**
-   * @todo Remove system_theme_default().
-   */
-  public function themeSetDefault() {
-    module_load_include('admin.inc', 'system');
-    return system_theme_default();
-  }
-
-  /**
    * #post_render_cache callback; sets the "active" class on relevant links.
    *
    * This is a PHP implementation of the drupal.active-link JavaScript library.
diff --git a/core/modules/system/lib/Drupal/system/Controller/ThemeController.php b/core/modules/system/lib/Drupal/system/Controller/ThemeController.php
index ece04d5..4e9b9b4 100644
--- a/core/modules/system/lib/Drupal/system/Controller/ThemeController.php
+++ b/core/modules/system/lib/Drupal/system/Controller/ThemeController.php
@@ -42,18 +42,18 @@ public function disable(Request $request) {
       if (!empty($themes[$theme])) {
         // Do not disable the default or admin theme.
         if ($theme === $config->get('default') || $theme === $config->get('admin')) {
-          drupal_set_message(t('%theme is the default theme and cannot be disabled.', array('%theme' => $themes[$theme]->info['name'])), 'error');
+          drupal_set_message($this->t('%theme is the default theme and cannot be disabled.', array('%theme' => $themes[$theme]->info['name'])), 'error');
         }
         else {
-          theme_disable(array($theme));
-          drupal_set_message(t('The %theme theme has been disabled.', array('%theme' => $themes[$theme]->info['name'])));
+          $this->get('theme_handler')->disable(array($theme));
+          drupal_set_message($this->t('The %theme theme has been disabled.', array('%theme' => $themes[$theme]->info['name'])));
         }
       }
       else {
-        drupal_set_message(t('The %theme theme was not found.', array('%theme' => $theme)), 'error');
+        drupal_set_message($this->t('The %theme theme was not found.', array('%theme' => $theme)), 'error');
       }

-      return new RedirectResponse(url('admin/appearance', array('absolute' => TRUE)));
+      return $this->redirect('system.themes_page');
     }

     throw new AccessDeniedHttpException();
@@ -81,17 +81,73 @@ public function enable(Request $request) {

       // Check if the specified theme is one recognized by the system.
       if (!empty($themes[$theme])) {
-        theme_enable(array($theme));
-        drupal_set_message(t('The %theme theme has been enabled.', array('%theme' => $themes[$theme]->info['name'])));
+        $this->get('theme_handler')->enable(array($theme));
+        drupal_set_message($this->t('The %theme theme has been enabled.', array('%theme' => $themes[$theme]->info['name'])));
       }
       else {
-        drupal_set_message(t('The %theme theme was not found.', array('%theme' => $theme)), 'error');
+        drupal_set_message($this->t('The %theme theme was not found.', array('%theme' => $theme)), 'error');
       }

-      return new RedirectResponse(url('admin/appearance', array('absolute' => TRUE)));
+      return $this->redirect('system.themes_page');
     }

     throw new AccessDeniedHttpException();
   }

+  /**
+   * Set the default theme.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   A request object containing a theme name.
+   *
+   * @return \Symfony\Component\HttpFoundation\RedirectResponse
+   *   Redirects back to the appearance admin page.
+   *
+   * @throws \Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException
+   *   Throws access denied when no theme is set in the request.
+   */
+  public function setDefaultTheme(Request $request) {
+    $config = $this->config('system.theme');
+    $theme = $request->query->get('theme');
+
+    if (isset($theme)) {
+      // Get current list of themes.
+      $themes = list_themes();
+
+      // Check if the specified theme is one recognized by the system.
+      if (!empty($themes[$theme])) {
+        // Enable the theme if it is currently disabled.
+        if (empty($themes[$theme]->status)) {
+          $this->get('theme_handler')->enable(array($theme));
+        }
+
+        // Set the default theme.
+        $config->set('default', $theme)->save();
+
+        \Drupal::service('router.builder')->setRebuildNeeded();
+
+        // 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 = $config->get('admin');
+        if ($admin_theme != 0 && $admin_theme != $theme) {
+          drupal_set_message($this->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'],
+            '%selected_theme' => $themes[$theme]->info['name'],
+          )));
+        }
+        else {
+          drupal_set_message($this->t('%theme is now the default theme.', array('%theme' => $themes[$theme]->info['name'])));
+        }
+      }
+      else {
+        drupal_set_message($this->t('The %theme theme was not found.', array('%theme' => $theme)), 'error');
+      }
+
+      return $this->redirect('system.themes_page');
+
+    }
+    throw new AccessDeniedHttpException();
+  }
+
 }
diff --git a/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
index 9e693fe..eba3a58 100644
--- a/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -11,50 +11,6 @@
 use Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException;

 /**
- * Menu callback; Set the default theme.
- */
-function system_theme_default() {
-  $request = \Drupal::request();
-  $theme = $request->get('theme');
-  if (!empty($theme)) {
-    // Get current list of themes.
-    $themes = list_themes();
-
-    // Check if the specified theme is one recognized by the system.
-    if (!empty($themes[$theme])) {
-      // Enable the theme if it is currently disabled.
-      if (empty($themes[$theme]->status)) {
-       theme_enable(array($theme));
-      }
-      // Set the default theme.
-      \Drupal::config('system.theme')
-        ->set('default', $theme)
-        ->save();
-
-      \Drupal::service('router.builder')->setRebuildNeeded();
-
-      // 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.
-      $admin_theme = \Drupal::config('system.theme')->get('admin');
-      if ($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'],
-          '%selected_theme' => $themes[$theme]->info['name'],
-        )));
-      }
-      else {
-        drupal_set_message(t('%theme is now the default theme.', array('%theme' => $themes[$theme]->info['name'])));
-      }
-    }
-    else {
-      drupal_set_message(t('The %theme theme was not found.', array('%theme' => $theme)), 'error');
-    }
-    return new RedirectResponse(url('admin/appearance', array('absolute' => TRUE)));
-  }
-  throw new AccessDeniedHttpException();
-}
-
-/**
  * Recursively check compatibility.
  *
  * @param $incompatible
diff --git a/core/modules/system/system.routing.yml b/core/modules/system/system.routing.yml
index 599f9c1..e7998a9 100644
--- a/core/modules/system/system.routing.yml
+++ b/core/modules/system/system.routing.yml
@@ -320,7 +320,7 @@ system.theme_set_default:
   path: '/admin/appearance/default'
   defaults:
     _title: 'Set as default theme'
-    _content: '\Drupal\system\Controller\SystemController::themeSetDefault'
+    _content: '\Drupal\system\Controller\ThemeController::setDefaultTheme'
   requirements:
     _permission: 'administer themes'
     _csrf_token: 'TRUE'