There's no.css file specific to viewforum_body.html (at leats not something I can immediately find) .
I think it could be done easily with an extension.
Use an event to add a.css file to be used with viewforum.body.html
hmm, just thought about that and it won't work (no event available - I checked), but it could actually be added to the Red title for new topic and new reply extension (using overall_header_head_append) - or just make it a separate extension.
Disclaimer: I didn't try it - if someone else does : don't test it on a board you don't want to hurt