THEMES tab in the Gmail Settings window.
Goodbye to good’old lain-vanilla blue-and-white Gmail!
Now you can select one of the 30 (and counting!) available skins for your Gmail page.
(Screenshot courtesy of Nirmal)
However, at this writing Themes are not available for every Gmail account. For example, I have it on one of my Gmail accounts but not on the other. If you cannot see the THEMES tab inside your SETTINGS window, be patient. It’ll be there soon enough.