Sometimes it's useful to add body classes based on the current Drupal user's roles.

This article shows how to do this by adding or modifying a template_preprocess_html() implementation in your .theme file. No twig modifications required.

Step 1: implement template_preprocess_html()

In your theme's MYTHEME.theme file, add the following preprocess function:

 * Implements template_preprocess_html().
function MYTHEME_preprocess_html(&$variables) {
  $roles = \Drupal::currentUser()->getRoles();
  foreach ($roles as $role) {
    $variables['attributes']['class'][] = "role-{$role}";

→ Be sure to replace MYTHEME with the name of your own theme.
→ Be sure to clear your caches.

Assuming the current user has the administrator role, the body classes for each of the pages this user loads will now also contain:

  • role-authenticated
  • role-administrator

Step 2: add styles

You can now add styles targeted at specific user roles.

We use this technique to render certain administrative / debugging styles for admin users.
Here's an example: highlighting images without alt descriptions:

.role-administrator img:not[alt],
.role-administrator img[alt=""] {
  border: solid red 2px;

Further resourses