I noticed tonight that Google has addded Themes to GMail and I have to say, they're really cool. To change from the default theme all you need to do is go to the settings area in Gmail and then click on the new Themes tab.
Here's a preview of some of my favourites.
The Ninja theme is currently my favourite. I just can't seep past having shurikens instead of normal stars. Yes, I'm easily pleased!!
I recommend checking it out if you use Gmail :)