Add theme settings field to theme-settings.php in d8 is not working for me
Try to remove this code:


  if (isset($form_id)) {
    return;


The hook interface of hook_form_system_theme_settings_alter is hook_form_system_theme_settings_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state).

You can remove the third parameter:


 $form_id = NULL