The first time, I went step by step through the bbpress.css file and made my styling changes. The first time lasted about two-three weeks and an upgrade trashed the changes by overwriting the file with the default.
The second time I installed bbPress on my local development site, went through and made my changes. I then uploaded the file to the live server. To my knowledge it has not been upgraded but a couple of weeks in and the install reverts to it’s default. A quick scan of the bbpress.css shows none of the changes I made.