diff --git c/core/modules/system/lib/Drupal/system/Controller/ThemeController.php w/core/modules/system/lib/Drupal/system/Controller/ThemeController.php
index 7cb00c8..39749e8 100644
--- c/core/modules/system/lib/Drupal/system/Controller/ThemeController.php
+++ w/core/modules/system/lib/Drupal/system/Controller/ThemeController.php
@@ -12,6 +12,7 @@
 use Drupal\Core\Access\CsrfTokenGenerator;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException;
+use Symfony\Component\HttpFoundation\RedirectResponse;
 
 /**
  * Controller for theme handling.
@@ -55,7 +56,8 @@ public function disable(Request $request) {
         drupal_set_message($this->t('The %theme theme was not found.', array('%theme' => $theme)), 'error');
       }
 
-      return $this->redirect($this->urlGenerator()->generateFromPath('admin/appearance', array('absolute' => TRUE)));
+      // @todo switch to $this->redirect when admin/appearance converted.
+      return new RedirectResponse($this->urlGenerator()->generateFromPath('admin/appearance', array('absolute' => TRUE)));
     }
 
     throw new AccessDeniedHttpException();
@@ -91,7 +93,8 @@ public function enable(Request $request) {
         drupal_set_message($this->t('The %theme theme was not found.', array('%theme' => $theme)), 'error');
       }
 
-      return $this->redirect($this->urlGenerator()->generateFromPath('admin/appearance', array('absolute' => TRUE)));
+      // @todo switch to $this->redirect when admin/appearance converted.
+      return new RedirectResponse($this->urlGenerator()->generateFromPath('admin/appearance', array('absolute' => TRUE)));
     }
 
     throw new AccessDeniedHttpException();
@@ -153,7 +156,8 @@ public function defaultTheme(Request $request) {
       else {
         drupal_set_message($this->t('The %theme theme was not found.', array('%theme' => $theme)), 'error');
       }
-      return $this->redirect($this->urlGenerator()->generateFromPath('admin/appearance', array('absolute' => TRUE)));
+      // @todo switch to $this->redirect when admin/appearance converted.
+      return new RedirectResponse($this->urlGenerator()->generateFromPath('admin/appearance', array('absolute' => TRUE)));
     }
     throw new AccessDeniedHttpException();
   }