diff -u b/adminimal_theme.theme b/adminimal_theme.theme
--- b/adminimal_theme.theme
+++ b/adminimal_theme.theme
@@ -34,7 +34,7 @@
 /**
  * Implements hook_form_system_theme_settings_alter().
  */
-function adminimal_theme_form_system_theme_settings_alter(&$form, \Drupal\Core\Form\FormStateInterface &$form_state, $form_id = NULL) {
+function adminimal_theme_form_system_theme_settings_alter(&$form, FormStateInterface &$form_state, $form_id = NULL) {
   // Work-around for a core bug affecting admin themes. See issue #943212.
   if (isset($form_id)) {
     return;