Unlike other webmail services, Gmail has never provided too many customization options and users had to rely on themes created for third-party extensions like Stylish, which usually slowed down Gmail and weren't resilient to code changes.
Gmail's help center 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, 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?
{ Thanks, Niranjan and Pascal . }