Custom template for 404 error pages
Yet another solution from this discussion that works without panels or other custom modules.

In your theme folder locate template.php and write this code. Replace YOURTHEME with the name of your theme:

function YOURTHEME_preprocess_page(&amp;$vars) {
  $header = drupal_get_http_header('status'); 
  if ($header == '404 Not Found') {     
    $vars['theme_hook_suggestions'][] = 'page__404';
  }
}


Whenever 404 occurs your page--404.tpl.php will be used. Remember to put "--" not "__" in your template file name.