WordPress 3.0: Activating the Background Editor
From the blog:
Adding a color or image to your Web site’s background is not a very complicated thing to do if you are familiar with CSS. Luckily for those out there who know nothing about CSS, WordPress 3.0 introduces a simple admin user-interface that makes it extremely simple to select a color or image for your background. There are even controls for image position, repeating and attachment.