diff --git a/core/includes/theme.maintenance.inc b/core/includes/theme.maintenance.inc
index b98d28a..994a662 100644
--- a/core/includes/theme.maintenance.inc
+++ b/core/includes/theme.maintenance.inc
@@ -6,6 +6,7 @@
  */
 
 use Drupal\Component\Utility\Unicode;
+use Drupal\Core\Theme\MissingThemeDependencyException;
 use Drupal\Core\Site\Settings;
 
 /**
@@ -92,9 +93,21 @@ function _drupal_maintenance_theme() {
       $theme_handler->addTheme($themes[$ancestor]);
     }
   }
+  // Ensure that at least the maintenance page has a working theme where all of
+  // the parent themes have been enabled.
+  $get_active_theme = function () use ($theme_init, $themes, $theme_handler, $custom_theme, &$get_active_theme) {
+    try {
+      return $theme_init->getActiveTheme($themes[$custom_theme]);
+    }
+    catch (MissingThemeDependencyException $e) {
+      $theme_handler->install([$e->theme]);
+      $themes = $theme_handler->rebuildThemeData();
+      return $get_active_theme();
+    }
+  };
   // @todo This is just a workaround. Find a better way how to handle themes
   //   on maintenance pages, see https://www.drupal.org/node/2322619.
-  \Drupal::theme()->setActiveTheme($theme_init->getActiveTheme($themes[$custom_theme], array_reverse($base_theme)));
+  \Drupal::theme()->setActiveTheme($get_active_theme(), array_reverse($base_theme));
   // Prime the theme registry.
   Drupal::service('theme.registry');
 }
diff --git a/core/lib/Drupal/Core/Theme/MissingThemeDependencyException.php b/core/lib/Drupal/Core/Theme/MissingThemeDependencyException.php
new file mode 100644
index 0000000..b28b652
--- /dev/null
+++ b/core/lib/Drupal/Core/Theme/MissingThemeDependencyException.php
@@ -0,0 +1,24 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Theme\MissingThemeDependencyException.
+ */
+
+namespace Drupal\Core\Theme;
+
+/**
+ * Exception to be thrown when base theme for installed theme is not installed.
+ *
+ * @see \Drupal\Core\Theme\ThemeInitialization::getActiveThemeByName().
+ */
+class MissingThemeDependencyException extends \Exception {
+
+  public $theme;
+
+  public function __construct($message, $theme) {
+    parent::__construct($message);
+    $this->theme = $theme;
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Theme/ThemeInitialization.php b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
index 95f27a9..af4b2c6 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -109,6 +109,9 @@ public function getActiveThemeByName($theme_name) {
     $ancestor = $theme_name;
     while ($ancestor && isset($themes[$ancestor]->base_theme)) {
       $ancestor = $themes[$ancestor]->base_theme;
+      if (!$this->themeHandler->themeExists($ancestor)) {
+        throw new MissingThemeDependencyException(sprintf('Base theme %s has not been installed.', $ancestor), $ancestor);
+      }
       $base_themes[] = $themes[$ancestor];
     }
 
diff --git a/core/lib/Drupal/Core/Theme/ThemeInitializationInterface.php b/core/lib/Drupal/Core/Theme/ThemeInitializationInterface.php
index f2b1547..cd5c1be 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitializationInterface.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitializationInterface.php
@@ -34,6 +34,9 @@ public function initTheme($theme_name);
    *
    * @return \Drupal\Core\Theme\ActiveTheme
    *   An active theme object instance for the given theme.
+   *
+   * @throws \Drupal\Core\Theme\MissingThemeDependencyException
+   *   Thrown when base theme for installed theme is not installed.
    */
   public function getActiveThemeByName($theme_name);
 
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 3d9f129..7d18d4f 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -22,6 +22,7 @@
 use Drupal\Core\Extension\ModuleHandler;
 use Drupal\Core\Url;
 use Drupal\Core\Block\BlockPluginInterface;
+use Drupal\Core\Theme\MissingThemeDependencyException;
 use Drupal\user\UserInterface;
 use Symfony\Component\HttpFoundation\RedirectResponse;
 use GuzzleHttp\Exception\RequestException;
@@ -524,8 +525,25 @@ function system_page_attachments(array &$page) {
     $page['#attached']['library'][] = 'system/admin';
   }
 
+  // Band-aid.
+  /** @var \Drupal\Core\Theme\ThemeManagerInterface $theme_manager */
+  $theme_manager = \Drupal::theme();
+  /** @var \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler */
+  $theme_handler = \Drupal::service('theme_handler');
+  $get_active_theme = function () use ($theme_manager, $theme_handler, &$get_active_theme) {
+    try {
+      $active_theme = $theme_manager->getActiveTheme();
+      return $active_theme;
+    }
+    catch (MissingThemeDependencyException $e) {
+      $theme_handler->install([$e->theme]);
+      $themes = $theme_handler->rebuildThemeData();
+      return $get_active_theme();
+    }
+  };
+
   // Attach libraries used by this theme.
-  $active_theme = \Drupal::theme()->getActiveTheme();
+  $active_theme = $get_active_theme();
   foreach ($active_theme->getLibraries() as $library) {
     $page['#attached']['library'][] = $library;
   }
diff --git a/core/themes/classy/classy.info.yml b/core/themes/classy/classy.info.yml
index fb5cb3e..7e74173 100644
--- a/core/themes/classy/classy.info.yml
+++ b/core/themes/classy/classy.info.yml
@@ -4,7 +4,6 @@ description: 'A base theme with sensible default CSS classes added. Learn how to
 package: Core
 version: VERSION
 core: 8.x
-base theme: false
 
 libraries:
   - classy/base