You Must have Known This By Now. I was just checking my mails today and WOW !!. Something looked different in My Gmail windows and I saw the color was changed to a lovely Sky Blue. I was amazed and looked around various blogs to find out about this great Update from Gmail.
Preview:
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. !!!!