public static function definePrestoThemeAsDefault() {
    // Set presto_theme as default.
    Drupal::configFactory()
      ->getEditable('system.theme')
      ->set('default', 'presto_theme')
      ->save();

    // Set seven as admin theme.
    Drupal::configFactory()
      ->getEditable('system.theme')
      ->set('admin', 'seven')
      ->save();
  }