So you want to install a custom theme to your Xbox One?

Step 1: Find the image you want and save it to a USB or DLNA source device

Step 2: Connect the USB with the background to console OR connect a source through DLNA

Step 3: Launch Media Player

Step 4: Navigate to source of your choosing (USB/DLNA)

Step 5: Navigate to folder/file of your choosing

Step 6: Select a jpg/png sized 1920/1080

Step 7: While viewing the image full screen, press the menu button

Step 8: Select "Set as background"

Step 9: Background is now set, you're done

Step 10: You're done, congratulations!