Hi Chaiwa,
It is not possible to add a watermark or modify the background within forms, though you can always add images to make the screen more engaging. You can propose the watermark idea on Dimagi's Uservoice site:
https://dimagi.uservoice.com/
It is, however, possible to change the CommCare icon you see when logging into the app and on the main screen by uploading icons on the Application Settings (this requires an Advanced Software Plan).
Also - if it feels like there is too much blank space on a screen you could use question groups to show more than one question on a screen.
Hope this helps,
Jeremy