How to Apply a Custom Theme
Connect a USB to console / Connect a source through DLNA
Launch Media Player (either directly or through Settings>My Xbox > My background > Custom image)
Navigate to source of your choosing (USB/DLNA)
Navigate to folder/file of your choosing
Select the image (Must be jpg/png format and no larger than 3MB) – see it full screen.
Once you are viewing the image full screen, hit the menu button.
Select “Set as background”
You will be sent back to the full screen view of the image. Hit nexus, see the image on Home.
How to make your own
You’ll need Photoshop to open the template and you can download it here. Alternatively you can preview your 1920 x 1080 images here to see how they would look on your dash.
Credit to r/XboxThemes for the instructions.