cache_ui-1.0.0/cache_ui.module

<?php

/**
 * Implements hook_preprocess_HOOK() for menu-local-action templates.
 */
function cache_ui_preprocess_menu_local_action(array &$variables) {
  if ($variables['element']['#link']['title'] === 'Purge Bin') {
    $variables['link']['#options']['attributes']['class'][] = 'button--secondary';
  }
}