bbPress.org

I suggest you use the Firefox browser and install the Web Developer toolbar from Chris Pederick. It will allow you to see what CSS classes apply to the content on the screen, then make changes to the live site to preview any changes.

http://en-us.www.mozilla.com/en-US/products/firefox/

https://addons.mozilla.org/en-US/firefox/addon/60

Those two things will enable you to figure this out.