I made a website years ago in PHP which had an RGB picker that would recolour the entire website; CSS, images and all.
Actually I made a .NET forum relatively more recently where each user could choose their own colours and border graphics and each of their posts would be styled and recoloured accordingly.
There were a lot of fun to do but they were hobby projects that I never properly finished, but as a concept it looked really nice.
Now, translating this to Orchard / MVC it'd be pretty easy to set up controller actions to pipe out dynamic CSS and images.
As for setting up background images in different zones, I could actually set that up already with Mechanics / Media Garden. I've already got background images for the content zone working in a site, that you can set for the whole site or customize per page.
So it'd be a nice place to end up at, but to be honest I have way too many other things to get done right now to get involved in anything like this :)