If you've ever wanted to make Gmail look just so for your personal tastes, today is your lucky day: Gmail has added a new "Create your own theme" option and tool where you can select background images and then choose your theme's colours.You'll find the new option in the Themes tab in Settings. Clicking on it brings up an easy palette chooser for text, link, and background colours. You can also select a Picasa image or upload a new one for the background. Both the main body and the footer images are customisable.
I did the theme shown here in about five minutes (but will no doubt go back to play with it). It won't likely change your life, but if you spend a lot of time in your inbox, you've now got the option to give it a new paint job or wallpaper.
Custom background image themes [Gmail Blog]