When Google unveiled the new look of Gmail back in November, they added lots of new HD themes. Users have asked Google for the ability to have custom backgrounds in Gmail. Over the next couple of days, Gmail will be rolling out the Custom Themes section to users over the next couple of days.
Google have created a video showing the different examples between the Light and Dark option:
Users can upload their own photos or use photos from their Google+ account. Or you can use the Featured photos if you don’t have any photos that you like.
Whether you want to bask on a tropical island or simply reminisce about last year’s holiday with the family, your Gmail can now be customized to your mood. Find your perfect image and make your Gmail your own with custom themes.
(Source: Gmail Blog)