ah, ok, you want to change your design dynamicly.
I dont understand why you use a plug in to change the look of the design.
the way I did is, I just took a link. mypage.php?page=1
then I set the trigger to $_GET['page']
Before I created 2 diffrent looking DW themes converted to FB themes. they both must contain the same edit able regions (names).
the you choose default for page 1
and the other design for page 2
to change the font size by a customer, you maybe should trigger the CSS.
Do you want to create something like myspace where the useres can change the look of ther page? I thing then you should do it with a customer backend.
I hope i understand you correctly :-)