Add HTTP status header as a body class

template.php

<?php

/**
 * Implements hook_preprocess_html().
 */
function <theme>_preprocess_html(&$variables)
{
  $header = drupal_get_http_header('status');
  if ($header && preg_match('`^[0-9]+`', $header, $matches)) {
    switch ($matches[0]) {
      case 403:
      case 404:
        $variables['classes_array'][] = 'http-status-'.$matches[0];
    }
  }
}