Issue #3148025 by Project Update Bot, iamdroid: Automated Drupal 9 compatibility fixes

theme-settings.php

 <?php
 
+use Drupal\Core\Form\FormStateInterface;
+
 /**
  * Implements hook_form_system_theme_settings_alter().
  */
-function material_base_form_system_theme_settings_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id = NULL) {
+function material_base_form_system_theme_settings_alter(&$form, FormStateInterface $form_state, $form_id = NULL) {
   // Work-around for a core bug affecting admin themes. See issue #943212.
   if (isset($form_id)) {
     return;