Following the footsteps of Google's personalized homepage, Gmail added themes. Once the new option is enabled in your account (as usually, Gmail's new features are slowly rolled out), you'll find a tab named "Themes" in the settings page and you'll be able to choose from 30 themes. "We wanted to go beyond simple color customization, so out of the 30 odd themes we're launching today, there's a shiny theme with chrome styling, another one that turns your inbox into a retro notepad, nature themes that change scenery over time, weather driven themes that can rain on your mailbox, and fun characters to keep you in good company," mentions Gmail's blog.
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.