diff -u b/core/includes/errors.inc b/core/includes/errors.inc
--- b/core/includes/errors.inc
+++ b/core/includes/errors.inc
@@ -288,10 +288,20 @@
 
 /**
  * Returns the current error level.
+ *
+ * This function should only be used to get the current error level pre
+ * DRUPAL_BOOTSTRAP_KERNEL or before Drupal is installed. In all other
+ * situations the following code is preferred:
+ * @code
+ * Drupal::config('system.logging')->get('error_level');
+ * @endcode
+ *
+ * @return string
+ *   The current error level.
  */
 function _drupal_get_error_level() {
   try {
-    return config('system.logging')->get('error_level');
+    return Drupal::config('system.logging')->get('error_level');
   }
   catch (Exception $e) {
     // During very early install the cache_config table does not exist.
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
@@ -52,10 +52,10 @@
     //   otherwise, as it looks generic enough and way more user-friendly.
     $custom_theme = variable_get('maintenance_theme');
     if (!$custom_theme)  {
-      $config = config('system.theme');
+      $config = Drupal::config('system.theme');
       // A broken install might not return an object.
       if (is_object($config)) {
-        $custom_theme = config('system.theme')->get('default');
+        $custom_theme = $config->get('default');
       }
     }
     if (!$custom_theme)  {