In this tuto, I'll share with you how to add body classes to 404 and 403 error pages in Drupal 8.
/**
* @param $variables
*/
function THEMENAME_preprocess_html(&$variables)
{
$statusCode = Drupal::request()->query->get('_exception_statuscode');
if (isset($statusCode) and (($statusCode == 404) or ($statusCode == 403))) {
$variables['attributes']['class'][] = 'page-' . $statusCode;
}
}
