Gmail help centre provides an interesting tidbit 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."
Gmail also adjusted the default theme,you can get ur theme by following dis step
Gmail --> Settings-->Theme.
but you can always go back to the previous theme by selecting "Classic" in the settings page. I can't decide between Contrast Black, Cherry Blossom, Graffiti and Ninja. What's your favorite theme?