diff --git c/core/modules/system/lib/Drupal/system/Controller/SystemController.php w/core/modules/system/lib/Drupal/system/Controller/SystemController.php
index 0b4a33c..d2261d9 100644
--- c/core/modules/system/lib/Drupal/system/Controller/SystemController.php
+++ w/core/modules/system/lib/Drupal/system/Controller/SystemController.php
@@ -174,7 +174,7 @@ public function systemAdminMenuBlockPage() {
    * Returns a theme listing.
    *
    * @return string
-   *  A HTML string of the theme listing page.
+   *   An HTML string of the theme listing page.
    */
   public function themesPage() {
     $config = $this->config('system.theme');
@@ -297,7 +297,15 @@ public function themesPage() {
       $theme_group_titles['disabled'] = format_plural(count($theme_groups['disabled']), 'Disabled theme', 'Disabled themes');
     }
 
-    uasort($theme_groups['enabled'], array($this, 'systemSortThemes'));
+    uasort($theme_groups['enabled'], function($a, $b) {
+        if ($a->is_default) {
+          return -1;
+        }
+        if ($b->is_default) {
+          return 1;
+        }
+        return strcasecmp($a->info['name'], $b->info['name']);
+      });
     $this->moduleHandler()->alter('system_themes_page', $theme_groups);
 
     $build = array();