[Plugin: bbPress Custom CSS File] "Theme directory"

Just a bit confused what you mean by “theme directory” — I’m using Genesis/Prose, along with Genesis Extend. I don’t have another bbPress theme installed. So where should I put the bbpress.css file?
– In the Genesis folder?
– in the Prose folder?
– in the Genesis Extend folder?
– somewhere else?

Thanks in advance!

http://wordpress.org/extend/plugins/bbpress-custom-css-file/