Google have released another feature to help improve your Gmail experience aesthetically. You can now in addition to using Color Themes and HD themes add custom themes. Custom themes are available in either Light or Dark and you can change the background image. Which means when you login to use Gmail you can now personalize and make it all your own. So improve your visuals in Gmail by adding pics of your dog, cat or just about anything. If you link an image you find on the net or maybe even find on Pinterest you can link back to that image and have it displayed in your Gmail.
How to create a Custom Theme in Gmail
1. login to Gmail
2. Click on the cog icon
3. Choose Themes, scroll down and select custom themes
4. To add your own photos select Change your backgroud image
5. You can now choose from my photos, photos from phone, upload photos, paste a URL and recently selected.
6. Choose the image you want displayed
7. You're done
Source: Gmail blog
Video below for the visual on how it's done.