Custom submit handler for theme settings form
From Drupal 8: Use Custom Submit Handler for Theme Settings Form:

Use this method to add custom submit handler for theme settings form in drupal 8.

/**
 * Implements hook_form_system_theme_settings_alter().
 */
function MY_THEME_form_system_theme_settings_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state) {
  $theme_file = drupal_get_path('theme', 'MY_THEME') . '/MY_THEME.theme';
  $build_info = $form_state-&gt;getBuildInfo();
  if (!in_array($theme_file, $build_info['files'])) {
    $build_info['files'][] = $theme_file;
  }
  $form_state-&gt;setBuildInfo($build_info);

  $form['#submit'][] = 'MY_THEME_form_system_theme_settings_submit';
}

function MY_THEME_form_system_theme_settings_submit(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state) {
  // TODO: Extra submssio logic.
  // This submit handler will be called before default submit handler for this form.
}