A small module for a quick start on theming the "Not found" and "Access denied" error pages. Adds multiple helper theme hooks for error 404 and 403 pages:

• Adds error404 and error403 body classes
• Adds theme hook suggestions such as: page--4xx.html.twig, page--403.html.twig and page--404.html.twig

Enable the module, clear the cache and you are good to go!