Following the footsteps of Google’s personalized homepage, Gmail has themes too. Gmail’s new features are slowly rolled out, So once this new option is enabled in your account. You’ll find a tab named "Themes" in the settings page and you’ll be able to choose from 30 themes. Gmail is officially riding the interface customization train by offering 30-some new themes to spice up your inbox.
From Gmail help. ‘Themes allow you to customize the look and feel of your Gmail account. To choose a theme, click a thumbnail on the Themes tab of your Settings page. 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.’