Dan

I would personally do this through CSS. I would create a CSS class within your active themes style sheet with the same name as the category name. If the category name contained spaces etc. I would write a quick function to add hyphens or something similar to ensure its was a CSS valid name.

Apply the above CSS class to the category template theme body/wrapper tag for example and the changes should take effect for each category. Those without a custom background should retain the default style.

if you wish to make some more elaborate changes such as overriding some base styles you could always create a custom category css file and dynamically load the CSS file based upon the localcategory.name within the pageload method of the category template.

Hopefully the above helps?