How to load theme style(s) in CKEditor?
Here is one way to do it... not sure if there are others (admin based):

/**
 * Implements hook_ckeditor_css_alter().
 *
 * Injects our CSS sheets anytime CKEditor has loaded.
 *
 * @param array $css
 * @param Drupal\editor\Entity\Editor $editor
 */
function mymodule_ckeditor_css_alter(array &amp;$css, Editor $editor) {
  if (!$editor-&gt;hasAssociatedFilterFormat()) {
    return;
  }

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

  if (in_array($editor-&gt;getFilterFormat()-&gt;id(), $known_formats)) {
    $css[] = drupal_get_path('theme', 'mytheme') . '/build/css/style.css';
  }
}