Have you tried Gmail's High Score Theme?

I absolutely love the High Score theme they released for Gmail last month. You can read the post over at the gmail blog. Not only does it have a retro 8-bit feel to it, it changes from blue background during the day to black background at night. The Google Talk gadget looks like an arcade console and even has little icons that look like space invaders. To install this theme on your own Gmail, log into your account, click on settings and then click on themes. You will see High Score listed near the bottom, click it and you're set!