Is There A Style Bridge to WordPress?
If I want to have a WordPress BLOG and a PHPBB forum on the same site and have them share the same styles, how can I do that?
You could simply link them both to the same style sheet. That style sheet can be anywhere. Of course, any classes, ids etc. that are referencing would need to be in the templates of both sections, but as long as that is covered, there's no problem.
You mean just have one CSS page for the two systems?
Can it be that easy?
Sure, though by default they will have their own style sheets in different places, but you can always change that. Set up your own stylesheet and direct them both to that. I don't see any problem with that.