Hi, that's not a bad idea, we already dynamically generate the body class, so I added the response code in there as well. https://github.com/NodeBB/NodeBB/commit/379a15632850e4ed0d7982cea2b5506ce06b7741.
If a page is 404 the body will have page-status-404
if there is an internal error it will have page-status-500