I new to PHP-Nuke, but I'll try to pose some idea...

It seems like you went about it the right way - looking in the *.css files.

So, maybe there's a way to put a style block of code in the /Forum/index.php or some other file(?)

Sorry I am not of much help. I was going to suggest what you have already tried. [Note: Sometimes the cache can be stubborn too, and not update...]

B-truE