Hi Ole,
I think you'll find pages would be easier because, well, they're pages. (and css applies to a page).
The problem you're having with the popup is because you have to see the popup as "part" of the underlying page, and css applies to pages.
So if you want the popup to be different, probably the best way to handle that is to override the CSS settings of the form itself to new values. ie it's not as east as just "swapping to another theme" - you need specific css settings for that form.
cheers
Bruce