diff --git a/core/lib/Drupal/Core/Theme/ThemeInitialization.php b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
index 0dbd0c6..7fb7f23 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -99,7 +99,7 @@ public function getActiveThemeByName($theme_name) {
     $ancestor = $theme_name;
     while ($ancestor && isset($themes[$ancestor]->base_theme)) {
       $ancestor = $themes[$ancestor]->base_theme;
-      $base_themes[] = $themes[$ancestor];
+      array_unshift($base_themes, $themes[$ancestor]);
     }
 
     $active_theme = $this->getActiveTheme($themes[$theme_name], $base_themes);