function MyTheme_preprocess_page(&$vars, $hook)
{
   if (isset($vars['node'])) {
       $vars['theme_hook_suggestions'][] = 'page__' . str_replace('_', '--', $vars['node']->type);
    }
}

function MyTheme_preprocess_page(&$variables, $hook) {  
 $status = drupal_get_http_header("status");  
  if($status == "404 Not Found") {      
    $variables['theme_hook_suggestions'][] = 'page__404';
  }
 
    if($status == "403 Forbidden") {      
    $variables['theme_hook_suggestions'][] = 'page__403';
  }
}