[@netweb Keymaster 11 years, 10 months ago]

The next release of bbPress will allow you to customise the CSS based on the user role in each topic or reply so you should be able to do what your asking here 🙂

See https://bbpress.trac.wordpress.org/ticket/2692 for further details 🙂

Additionally also see this post on how to achieve the same results now 😉