Most that play with YaBB wind up altering the look greatly. It is a good thing and a bad thing at times. I had to hack up the perl to get everything usable on the Webtv end. The problem with that is that unless you don't mind busting into the perl every time you don't want to upgrade much.
BTW, if a Webtv user has problems with holding the login you need to make your cookie time less. I think the "stay logged in" deal from the year 2041 to something more realistic, like a year or two. Webtv chokes on outrageous cookie times.