Re: Would copying the theme of bbpress.org be illegal?
I think all bbPress themes are protected by GPL since they must call bbPress functions.
By GPL, you can modify, redistribute, or even almost change nothing to re-release as long as you don’t claim yourself is the only one copyright holder if you are not. Just don’t replace “Copyright YEAR HOLDERS”, but append new one with your name. Moreover, If someone asks for the source, you have no reasons to refuse the request.
PS. I am not sure I am totally correct.