D8 Preprocess classes by Role

/**
 * Implements hook_preprocess_html().
 */
function MYTHEME_preprocess_html(&$variables) {

  // Body classes for auth roles.
  if ($variables['logged_in']) {
    $user_roles = user_role_names(TRUE);
    foreach ($user_roles as $role => $title) {
      $variables['attributes']['class'][] = Html::cleanCssIdentifier('role--' . $role);
    }
  }
  
}