For example my BBPress pulls the top header image, navigation and footer from wordpress. It does not pull between the <head> </head> tags. But this allows me to control the nav and header in one place as well as footer, so that everything gets updated.
If you have your bbpress install setup to use wordpress functions it’s pretty easy to do.
As for CSS, I hate the way other people code css so I always end up ripping it all out and starting from scratch.