bbPress.org

You would have parent topics of “Main Theme” and “Secondary Themes” and then children of Main Theme as

Main Theme

— Installation

— Troubleshooting

— Themes

— Plugins

— Requests and Feedback

— Pimp your Press

and children of you Secondary Themes as

Secondary Themes

— Support Theme

— Crystal Theme

— Misty Theme

— Free Blank Theme

— Future King Theme

Actually, after rereading that, I’m NOT sure that does what you want. You don’t want subforums, you just want categories of forums. Having never done it, I’m not sure it would work that way if you did it as I described. Did you try it and if so, what was your result?