Sorry, i try to find it in the UCP. Also take a look into the modules (maybe it was disabled). But overall, i didn't found the place where user are able to select there style.
Situation: i host an 4.000 users "big" PHPbb2 (now 3) Forum and i though it would be cool to use the good old phpbb2 style (like we did before upgrading) so Users get an smooth update. But i also though it would be cool if some "freaks" are able to use the new style, but i didn't find the option where user can select there style, both (subsilver2 and prosilver) are active in the backend.
hope you can help me.