niobi-8.x-2.0-alpha4/modules/niobi_admin/niobi_admin.module

<?php

/**
 * Implements hook_element_info_alter().
 */
function niobi_admin_element_info_alter(array &$types) {
  // Attach our extra CSS for toolbar icons.
  if (isset($types['toolbar'])) {
    $types['toolbar']['#attached']['library'][] = 'niobi_admin/toolbar';
  }
}