@Christine
If they are not in a Usergroup that is allowed to use the Style, it will fallback to forum default in runtime.
Just like it would for not-existing styles or non-user selectable styles.

Thanks Amy and Kirby.

Nice hack! :)

Doesn't it work on existing styles? I couldn't find it on Style Settings...

/me installed.

Yes it does.
Do you run RC3?

installed on 3.5 gold.. it doesn't work properly :ermm:

What exactly does "not work properly"?

it's hard to explain... cause it behave in different way...
once i ticked style1 only for groupA, and style2 only for groupB -> groupA had style2 and groupB style1 :\

now i'm trying different solutions but it seems not to have any effects. i mean: whatever i choose, nothing changes.

I'm not sure if you know this or not, but when you manually change the style (via link), you can view the styles you don't allow the users of that group to view.

i know :) infact "groupA" is my group (admin), and "groupB" is for "unregistred" (so i use another browser to test if it works)