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 what's going on and Refreshed the page and got a message on the top which was pointing that Gmail have themes now.
Here is the picture.
Click This image.
Gmail's help center provides an interesting tab about the new themes: In some cases, you can also customize by location. Some themes change during the day, and we use the location information you provide to correctly time these changes with your local sunrise, sunset, and/or weather. If you select one of these themes, you'll see a Country/Region drop-down menu appear.