Don Punch: I can't give you anything specific related to PHPBB having never developed for it myself, but I can point you in the direction of FireBug, an add-on for FireFox. It is quite possibly the best web development tool around (and as a long-time Opera user, that's a big thing for me to say!).
What you'll be able to do with FireBug is first of all see what stylesheets are in use by the page in question. As well as that, you'll be able to directly 'select' the element you want to change and FireBug will spit out what styles are affecting it. This should help you narrow down exactly what you need to change to get it working. As well as that, FireBug allows you to edit the stylesheet directly without touching the online version, so you can see your changes instantly without screwing around with the live version.
Yes, I know this seems like an ad for FireBug, but it's free and it's definitely helped me out in my job.
Luke and Crollo: This is General Discussion so the post is valid. If you aren't going to help, don't post.