[MeNeedHelp]

You could insert this in the global.php file.
($hook = vBulletinHook::fetch_hook('style_fetch')) ? eval($hook) : false;

$numstyles = 3; // however many styles you have
$randstyle = rand(0, $numstyles * 1000);

if ($randstyle < 1000)
{
$vbulletin->userinfo['styleid'] = 1;
$styleid = 1;
}
else if ($randstyle < 2000)
{
$vbulletin->userinfo['styleid'] = 2;
$styleid = 2;
}
else if ($randstyle < 3000)
{
$vbulletin->userinfo['styleid'] = 3;
$styleid = 3;
}