Putting a style mandatory temporarily
 

We have several styles on our forum. Now we are going to have a special style for the holidays and I would like to put that style mandatory for every user.

That's not a big problem I guess* but the issue is that I would like it changed in such way* that after I disable the mandatory style* the user get's the style back which he had choosen before.

For example user 1 has now style 3* gets mandatory style 5 and when it's not mandatory anymore* he will automatically change back to style 3. If a user has style 1* he will get back style 1 and so on.

How can I do this so it works this way? I know it's possible because I've seen it working that way on several forums* I just don't know how.

I can put my special style mandatory by enabling it and disable the choice for users to use other styles I guess. But if I enable the choice option afterwards* to they get their own style back?

If not* what do I have to do* to get it working this way?