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First a wacky question: have you tried copying the default template to the other themes' folders? (as if it was a custom template?)
Then, two wackier suggestions:
1st Yes it's possible to do that kind of trick using CSS (not sure if CSS
only, but you'd be able to add a DIV by hacking your theme). Check
this thread for a discussion and some nice links.
2nd You can have a single template that behaves as many
Not sure about what you could use to alternate rendering, I gave a lame referer example
here. But you don't have to hack templates, IMHO.