diff --git a/core/includes/theme.maintenance.inc b/core/includes/theme.maintenance.inc
index b98d28a..6224854 100644
--- a/core/includes/theme.maintenance.inc
+++ b/core/includes/theme.maintenance.inc
@@ -82,19 +82,19 @@ function _drupal_maintenance_theme() {
   $theme = $custom_theme;
 
   // Find all our ancestor themes and put them in an array.
-  $base_theme = array();
+  $base_themes = array();
   $ancestor = $theme;
   while ($ancestor && isset($themes[$ancestor]->base_theme)) {
-    $base_theme[] = $themes[$themes[$ancestor]->base_theme];
+    $base_themes[] = $themes[$themes[$ancestor]->base_theme];
     $ancestor = $themes[$ancestor]->base_theme;
     if ($ancestor) {
-      // Ensure that the base theme is added.
+      // Ensure that the base theme is added and installed.
       $theme_handler->addTheme($themes[$ancestor]);
     }
   }
   // @todo This is just a workaround. Find a better way how to handle themes
   //   on maintenance pages, see https://www.drupal.org/node/2322619.
-  \Drupal::theme()->setActiveTheme($theme_init->getActiveTheme($themes[$custom_theme], array_reverse($base_theme)));
+  \Drupal::theme()->setActiveTheme($theme_init->getActiveTheme($themes[$custom_theme], $base_themes));
   // Prime the theme registry.
   Drupal::service('theme.registry');
 }
diff --git a/core/lib/Drupal/Core/Theme/ThemeInitializationInterface.php b/core/lib/Drupal/Core/Theme/ThemeInitializationInterface.php
index cd5c1be..a3b54df 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitializationInterface.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitializationInterface.php
@@ -57,8 +57,8 @@ public function loadActiveTheme(ActiveTheme $active_theme);
    * @param \Drupal\Core\Extension\Extension $theme
    *   The theme extension object.
    * @param \Drupal\Core\Extension\Extension[] $base_themes
-   *    An array of extension objects of base theme and its bases. It is ordered
-   *    by 'oldest first', meaning the top level of the chain will be first.
+   *   An array of extension objects of base theme and its bases. It is ordered
+   *   by 'next parent first', meaning the top level of the chain will be first.
    *
    * @return \Drupal\Core\Theme\ActiveTheme
    *   The active theme instance for the passed in $theme.
diff --git a/core/modules/system/src/Tests/Installer/StandardInstallerTest.php b/core/modules/system/src/Tests/Installer/StandardInstallerTest.php
index 184e07b..3c4011e 100644
--- a/core/modules/system/src/Tests/Installer/StandardInstallerTest.php
+++ b/core/modules/system/src/Tests/Installer/StandardInstallerTest.php
@@ -41,6 +41,21 @@ protected function setUpSite() {
   }
 
   /**
+   * {@inheritdoc}
+   */
+  protected function curlExec($curl_options, $redirect = FALSE) {
+    if ($redirect) {
+      // Ensure that we see the classy progress css on the page.
+      if (strpos($curl_options[CURLOPT_URL], 'do_nojs') !== FALSE) {
+        $this->assertRaw('themes/classy/css/components/progress.css');
+      }
+    }
+
+    return parent::curlExec($curl_options, $redirect);
+  }
+
+
+  /**
    * Ensures that the exported standard configuration is up to date.
    */
   public function testStandardConfig() {