The Official Gmail blog introduces Themes to Gmail.
If you use Gmail, you could modify the theme of Gmail in the "Themes tab under Setting" or click here
Some of the themes work similar to the iGoogle's themes that they will be different according to the time of your location. (They will ask you for your location when you choose the relevant themes)
These are the themes you can choose from. Currently, I'm using "Tea House"
I feel green. ahaha