I have to say, GMail themes are pretty cool! Some actually change based on the time of day. I'm "Phantasea" how about you?
For more information on GMail themes, check out their help section here:
Setting up a theme in your GMail box is easy - login, click "Settings" (upper right), select the theme of your choice, and set your location (some themes change throughout the day based on your location). I'll admit that I'm not usually a fan of this sort of add-on (who needs bells and whistles in an email platform?) but Google did a really good job with implementation - very seamless and tasteful.
