diff -u b/core/includes/theme.maintenance.inc b/core/includes/theme.maintenance.inc
--- b/core/includes/theme.maintenance.inc
+++ b/core/includes/theme.maintenance.inc
@@ -12,7 +12,7 @@
  * It also applies when the database is unavailable or bootstrap was not
  * complete. Seven is always used for the initial install and update
  * operations. In other cases, Bartik is used, but this can be overridden by
- * setting a "[system.theme][maintenance]" key in the $conf variable in settings.php.
+ * setting a "maintenance_theme" key in the $conf variable in settings.php.
  */
 function _drupal_maintenance_theme() {
   global $theme, $theme_key, $conf;
@@ -32,7 +32,7 @@
 
   // 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['system.theme']['maintenance']) ?: 'seven';
+    $custom_theme = (isset($conf['maintenance_theme']) ? $conf['maintenance_theme'] : 'seven');
   }
   else {
     // The bootstrap was not complete. So we are operating in a crippled
@@ -50,7 +50,7 @@
     //   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 = config('system.theme')->get('maintenance') ?: config('system.theme')->get('default') ?: 'bartik';
+    $custom_theme = variable_get('maintenance_theme', config('system.theme')->get('default'));
   }
 
   // Ensure that system.module is loaded.
diff -u b/sites/default/default.settings.php b/sites/default/default.settings.php
--- b/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -548,11 +548,11 @@
  * 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' key in $conf['system.theme']. The template file should also be copied into 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'.
  * Note: This setting does not apply to installation and update pages.
  */
-# $conf['system.theme']['maintenance'] = 'bartik';
+# $conf['maintenance_theme'] = 'bartik';
 
 /**
  * CSS/JS aggregated file gzip compression: