Gallbladder

Quick and simple way to add the new custom background selector to your WordPress 3.0 theme.
Seriously. That�s it. Just add that to the theme�s functions.php file.

Details

Okay, so your theme does need to have the normal wp_head() call in it. For those of you more CSS inclined, this basically creates CSS code for the body and adds that code to the head output directly. Voila, your theme gets styled.
Note that you will need to also not define your own background stuff in the theme for this to work. If the user tries to put in a solid color background and you define an image background, then the color won�t work or be visible over your image. Best to not put anything background related onto the body at all, in fact.

Customization

For those more inclined to customize things, there�s actually three parameters you can use:
Each of these are references to a callback function. If you use them, then you need to define your own callback functions to replace the default ones.
The header_callback function is what builds and outputs the CSS. The function takes no parameters, but it can use get_background_image() and get_background_color() to retrieve the necessary information. From this information, the function should produce and output (echo) the necessary