BuddyPress.org

There are a couple of files that can be used that bp looks for. You\’ll need to create them. In the themes, if you create custom.css in the /css directory it overrides the css for each theme. Also in the home theme or your custom theme if you create a file called /css/site-wide.css that overrides all css generated by bp.

That way you don’t need to modify the distributed css files and your upgrade experience will be much happier.