Smart Backgrounds is now available in Downloads.
This mod allows you to use different header backgrounds for each top category or page.
If an image file is found for a category or page, the image will be displayed as the header background;
if not, the default (home page) background image will be displayed.
The feature may also be applied to any other element on the page independently, by moving and modifying the code appropriately (examples given in the readme).
It may also be used to switch foreground images in any element based on current top category or main page (code given in the readme).
I don't have spare hosting space for a demo site, so it would be good if someone who installs this would post a link to see it in action.