linkit_custom_link-1.0.0-alpha1/linkit_custom_link.module

<?php

/**
 * @file
 * Module hooks.
 */

use Drupal\editor\Entity\Editor;

/**
 * Implements hook_ckeditor_css_alter().
 */
function linkit_custom_link_ckeditor_css_alter(array &$css, Editor $editor) {
  $css[] = \Drupal::service('extension.list.module')->getPath('linkit_custom_link') . '/css/linkit_custom_link.ckeditor.css';
}