How to modify styleid from a hook?

[Genent]

Hello everyone,
I would like to do the following: I have 3 styles defined in vBulletin, there is a default style, and two customized styles for two sub-forums. I have successfully set up the forum to show each subforum's customized style, but when visitors enter a subforum, then clicks on User CP, or Register, or anything that's outside that particular subforum, he gets the default style again.
I would like to store their last visited forum's style, and use that style when they click out of that subforum which has that style defined. I looked into global.php but style setup seems a bit weird to me, I don't know exactly which hook should i use, and what variable to modify.
Thanks for your help