(Drupal 8.x) Load theme CSS into CKEditor for certain formats

mytheme.theme

<?php
use Drupal\editor\Entity\Editor;

/**
 * Implements hook_ckeditor_css_alter().
 *
 * Original source: https://drupal.stackexchange.com/a/225532
 */
function mytheme_ckeditor_css_alter(array &$css, Editor $editor) {

  // Load front end theme CSS into CKEditor for certain formats.
  if (!$editor->hasAssociatedFilterFormat()) {
    return;
  }

  $known_formats = [
    'basic_html',
    'full_html'
  ];

  if (in_array($editor->getFilterFormat()->id(), $known_formats)) {
    // Add main theme stylesheet.
    $css[] = drupal_get_path('theme', 'mytheme') . '/css/app.css';
    // Add extra CSS for CKEditor.
    $css[] = drupal_get_path('theme', 'mytheme') . '/css/ckeditor.css';
  }
}