Drupal 8's CKEditor uses the Moono skin by default. In this tuto I'll show you how to create custom CKEditor Moono Dark skin.
By default CKEditor skin is like this:
Create ckeditor_moono_dark_skin module
Create ckeditor_moono_dark_skin.info.yml file:
name: CKEditor Moono Dark
type: module
description: Create Custom CKEditor Moono Dark Skin
core: 8.x
package: Codimth
dependencies:
  - ckeditor
Create ckeditor_moono_dark_skin.module file:
<?php
/**
 * @param array $settings
 */
function ckeditor_moono_dark_skin_editor_js_settings_alter(array &$settings) {
    foreach (array_keys($settings['editor']['formats']) as $text_format_id) {
        if ($settings['editor']['formats'][$text_format_id]['editor'] === 'ckeditor') {
            $settings['editor']['formats'][$text_format_id]['editorSettings']['skin'] = 'moono-dark,/libraries/moono-dark/';
        }
    }
}
Installation
Download the Moono Dark module, put it into the "libraries" folder.
Enable ckeditor_moono_dark_skin module.
Next steps
- Clear your Drupal 8 caches. To do this I use this Drush command: 
drush crif you don’t currently use Drush, I highly recommend using it, or the Drupal Console. - Now, go back to your site, and you should be able to see the ckeditor skin is changed as picture below.
 - I hope you found this article useful. let me know if you have any questions and I’ll be happy to answer them.
 - This code can be found and downloaded from https://github.com/codimth/ckeditor_moono_dark_skin.