I would love it if my wife could pull something like this for me. I can already picture my Seahawks tanks.
Use a wall poster of a favorite team as a background, so you will need to to cut the poster to fit the background.
As for gravel, I would use the favorite team color. Seahawks are bright green and navy, so I would use those two colors.
Then I would like to add more logos (in addition to the background), but I can't think of anything that can stay under water. You could buy some plastic helmets and scatter them on the floor of the tank. Wash them really well before you put them in. I see them in stores sometimes: NFL Football Helmets On The Cheap
I'm not sure how aquarium safe they are.
As for decor, I think I would add a glass football into the tank. glass football | eBay
I see so many things I can add to the tank for decor on ebay, but again I don't know if they are aquarium safe. I would only buy stuff that is 100% glass or certain plastic. Shot glasses and cups touch water all the time.. so use your best judgement.