Is there code or a plugin that would make it possible to add default backgrounds, like you can with default headers in Twenty ten. Seems like the same frame work for backgrounds and headers.
There's no default way to do that yet, though there is a Trac ticket. I found this (untested) that may work for you: http://www.devblog.fr/2011/05/16/plusieurs-fonds-personnalises-wordpress-add_custom_background/
It's also worth noting the 3.4's new way of defining a default background (still just one, though).
Here's the code:
(Note that you can also define front end, admin, and preview style callbacks, as well.)
If you can, add this snippet to your theme
Or if you have setup function for your theme, just add this call to it:
After you added it, new menu item "Background" will appear in your admin panel. At this page you can select custom background image or set background color: