First you beat the game. Then load the saved file for that game. Go into the blue Toad house now located in World 1. In here you can purchase five different backgrounds for your lower screen. They all cost Star Coins. To get the fift background, you have to get all of the star coins in the through-out all the levels.
Toady claims to get the fifth background you must collect all star coins. This is wrong. You only need to clear a path to every mushroom hut. Finish the game and a path to a blue mushroom hut will appear in World 1. It contains the starting touchscreen theme and three additional background patterns that can be purchased for 20 star coins each. A fifth theme can be bought once you've cleared paths to the mushroom huts in every world.