/**
 * Implements hook_system_themes_page_alter().
 */
function {{ machine_name }}_system_themes_page_alter(&$theme_groups) {
  foreach ($theme_groups as $state => &$group) {
    foreach ($theme_groups[$state] as &$theme) {
      // Add a foo link to each list of theme operations.
      $theme->operations[] = [
        'title' => t('Foo'),
        'url' => Url::fromRoute('system.themes_page'),
        'query' => ['theme' => $theme->getName()],
      ];
    }
  }
}
