diff --git a/core/includes/theme.maintenance.inc b/core/includes/theme.maintenance.inc
index bd90379..e87d8df 100644
--- a/core/includes/theme.maintenance.inc
+++ b/core/includes/theme.maintenance.inc
@@ -34,7 +34,7 @@ function _drupal_maintenance_theme() {
 
   // Install and update pages are treated differently to prevent theming overrides.
   if (defined('MAINTENANCE_MODE') && (MAINTENANCE_MODE == 'install' || MAINTENANCE_MODE == 'update')) {
-    $custom_theme = (isset($conf['maintenance_theme']) ? $conf['maintenance_theme'] : 'seven');
+    $custom_theme = isset($conf['system.theme']['maintenance']) ?: 'seven';
   }
   else {
     // The bootstrap was not complete. So we are operating in a crippled
@@ -52,7 +52,16 @@ function _drupal_maintenance_theme() {
     //   Stark otherwise. Since there is no low-level access to configuration
     //   currently, we only consult settings.php and fall back to Bartik
     //   otherwise, as it looks generic enough and way more user-friendly.
-    $custom_theme = variable_get('maintenance_theme');
+    if (isset($conf['system.theme']['maintenance'])) {
+      $custom_theme = $conf['system.theme']['maintenance'];
+    }
+    elseif (Drupal::config('system.theme')->get('default')) {
+      $custom_theme = Drupal::config('system.theme')->get('default');
+    }
+    else {
+      $custom_theme = 'bartik';
+    }
+
     if (!$custom_theme)  {
       $config = Drupal::config('system.theme');
       // A broken install might not return an object.
diff --git a/sites/default/default.settings.php b/sites/default/default.settings.php
index fe16cfc..c6596b1 100644
--- a/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -567,11 +567,12 @@
  * A custom theme can be set for the offline page. This applies when the site
  * is explicitly set to maintenance mode through the administration page or when
  * the database is inactive due to an error. It can be set through the
- * 'maintenance_theme' key. The template file should also be copied into the
- * theme. It is located inside 'core/modules/system/maintenance-page.tpl.php'.
+ * $conf['system.theme']['maintenance']. The template file should also be copied
+ * into the theme. It is located inside
+ * 'core/modules/system/maintenance-page.tpl.php'.
  * Note: This setting does not apply to installation and update pages.
  */
-# $conf['maintenance_theme'] = 'bartik';
+# $conf['system.theme']['maintenance'] = 'bartik';
 
 /**
  * CSS/JS aggregated file gzip compression: