bbPress.org

Yeah, assuming the themes have consistent HTML, in the header, put something like this:

<link rel="stylesheet" href="<?php if ($forum_id == 1) bb_stylesheet_uri(); elseif ($forum_id == 2) echo '/my-templates/kakumei-blue/style.css'; ?>" type="text/css" />

I didn’t test this, but I think that code should work.