I’m no expert on bbpress theming, but I got it working on my site.
The bbpress theme files are in the bbpress/templates/default folder.
Whatever you copy from this core bbp theme folder into your own theme folder will override the core bbp theme files. (I think).
Probably someone more knowing about bbpress theming should help you out further, but until then, I hope I might have gotten you going again.
This is a nice guide to what the individual theme files actually do: