class BackgroundProperties? Could have that keep an image URL and a colour and have a method on it to form and return a CSS background property specification:

background: #000000 url(http://blah);

The string version of that data to pass to the constructor from /customize/ can just be as simple as a color followed by whitespace followed by an optional URL. This also means that other kinds of special background could be allowed later such as “transparent”.

UI would allow the user to pick a colour or “transparent”, and if a colour is picked they can then choose between “solid color”, “pattern” or “other image”, where pattern is one of the new images. This mimicks roughly how it works in Windows while adding the transparent option. The preview in the customize interface itself (from which the “change” button will be clicked canjust be a slightly bigger version of the background-colour-box used in the colour picker, with the background properties applied to contents of that box.

I'm sure you already had something like this planned anyway, but whatever! I'm working on a layout right now which could use this. (Cleanly('s H| Sh)aven, finally.)