Browsing my email today and saw a new note at the top with options to set up Gmail Themes- a very

innovation- Currently I have mine set to planets and its awesome how the color scheme is integrated with the background image. Now only if we could change the background image
Also, I like how the images change on the location... but haven't seen how dynamic it is yet... ie- is it tied to weather, time of day, etc...
...And a little more searching answered my question:
Originally Posted by google
From Google
Themes
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. 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.