[eluser]sophistry[/eluser]
not a stupid question! it's actually very deep and gets right to the heart of building good code and separating the logic from the display.
but, one might say that you have a not-so-good question title!
try posting the meaning of your question in the title next time and you might get better feedback.
like this: " what is the best way to make user-switchable 'themes'? "
cheers.