Taken from The WordPress MU Manual
Your bbPress MU comes with two default theme installed (Kakumei and Kakumei blue). To extend available themes you’ll need to install extra themes on your site.
bbPress themes can be downloaded from bbPress Showcase
Installing New Themes
1. Download the new theme(s)
2. Unzip the file to extract all files into a folder on your hard drive
3. Launch FileZilla
4. Enter your domain name, your username and password. NOTE: the same username and password that you use to log into your cPanel.
5. Click Quickconnect
6. Locate and open up the folder on your computer that contains the new theme(s).
7. Open up your bbPress-templates folder for your site (located within forum folder of your public_html folder)
8. Select the new theme folders in the folder on your computer and drag it into your bb-templates folder on your site
9. Dragging the theme folders from your computer into theme folder of your site uploads these new themes to your bbPress forum however they aren’t enabled.
10. To change themes you need to log into your admin in bbPress.
11. Go to Design and click on the name of the theme you want to activate.