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.
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. Select the country you want, and then enter a city in the field provided. If you don’t enter a city, or enter an invalid one, we’ll set your location to the capital city of the country you selected. When you’re done, click Save.
When choosing a theme, you can tell whether it changes by looking at the lower right hand corner of the theme’s thumbnail image. Small icons indicate if the theme changes with the weather, time of day, or day of the week. Also, if you see a white corner it means that that the message list background in the theme is light; if it’s black, that means it’s dark.[source]