Question

When a user gets an Access Denied page, how can you use drupal_add_css to load a particular stylesheet? Is this possible?

Answer 1

It is possible. The following code can add a CSS file when Drupal returns a access denied page.
Take in mind that when Drupal returns such error, the URL shown in the browser doesn’t change; in example, if I am trying to access /admin/content, and I am not allowed to access that page, the shown URL doesn’t change. You need to check the headers output from Drupal to verify if it output the access denied header.

function custom_module_preprocess_page(&$variables) {
  if (preg_match('@HTTP/1\.[01]\x20+403[^a-zA-Z0-9]@', drupal_get_headers())) {
    $variables['css'] = drupal_add_css($css_file_to_add);
    $variables['styles'] = drupal_get_css();
  }
}

The equivalent for Drupal 7 is the following one.

function custom_module_preprocess_page(&$variables) {
  if (drupal_get_http_header('Status') == '403 Forbidden') {
    $variables['css'] = drupal_add_css($css_file_to_add);
    $variables['styles'] = drupal_get_css();
  }
}

The same code can be used for a preprocess function implemented by a theme.