Nov 22, 2008
I was just checking my mails and Something happened to My Gmail windows and I saw the color was changed to Sky Blue. I was like WTF!!! and Refreshed the page and got a message on the top which was pointing that Gmail have themes now. 
Themes in Gmail
Shiny Theme
Shiny Theme for Gmail!!!
Desk Theme
Zoozimp Theme
Zoozimps Theme for Gmail
Update : Some themes might look for your location and change accordingly but not sure what they are changing.
Gmail Themes Location
Gmail Themes : Change your location
In case you want to change the location preferences, look for a link right at the bottom of the themes page in settings.
Just check out your settings page and you should have Themes there. !!!!