diff -u b/core/modules/system/lib/Drupal/system/Form/ThemeAdminForm.php b/core/modules/system/lib/Drupal/system/Form/ThemeAdminForm.php
--- b/core/modules/system/lib/Drupal/system/Form/ThemeAdminForm.php
+++ b/core/modules/system/lib/Drupal/system/Form/ThemeAdminForm.php
@@ -29,19 +29,19 @@
     // Administration theme settings.
     $form['admin_theme'] = array(
       '#type' => 'details',
-      '#title' => t('Administration theme'),
+      '#title' => $this->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.'),
+      '#options' => array(0 => $this->t('Default theme')) + $theme_options,
+      '#title' => $this->t('Administration theme'),
+      '#description' => $this->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'),
+      '#value' => $this->t('Save configuration'),
     );
     return $form;
   }
@@ -50,7 +50,7 @@
    * {@inheritdoc}
    */
   public function submitForm(array &$form, array &$form_state) {
-    drupal_set_message(t('The configuration options have been saved.'));
+    drupal_set_message($this->t('The configuration options have been saved.'));
     $this->config('system.theme')->set('admin', $form_state['values']['admin_theme'])->save();
   }
 }