The attached example demonstrates how you can smoothly change a theme’s colour using jQuery and jQuery Color. I’m not a design guru by any means, but I think the idea is sound: develop your images to show the highlights and shadows using transparent PNGs and shift the colour of some background class. You could even shift a set of colour classes to get complimentary colours going on.

I get that this won’t work in IE6 and is limited in the visual effects that it can produce, but it’s an interesting toy to consider.