Google is now allowing users to share their custom Gmail themes, in a feature much like their Gmail Shelfie April Fool's joke (was it really a joke?). If you are using a custom theme in Gmail, simply go to Settings and under Themes, you'll see the new "Share your theme" option. You will be able to share with Gmail, Google+, or with a direct link.
Google will be sharing some of their favorite themes on Google+ on Thursdays.
Source: Gmail blog