You could try something like this:
in global.php, find:
Code:
// style specified in user profile
$styleid = $bbuserinfo['styleid'];
REPLACE WITH:
PHP Code:
if ($bbuserinfo['styleid'] == X AND $bbuserinfo['usergroupid'] != Y)
{
$styleid = $vboptions['styleid'];
}
else
{
// style specified in user profile
$styleid = $bbuserinfo['styleid'];
}
replace X with the styleid and Y with the usergroup that's allowed to view that style.
(untested)