The design can be changed in just two steps: 

1. Open the Minesweeper.World App

2. Open the "Game Settings" (gear in the upper right corner) 

3. Tap on the button to the right of the "Theme" option

4. Choose one of the different background images like Classic, Beach or Pizza

Note: Please note that some themes cost Rubies. If you do not have enough, you can fill them up by buying them in the Shop."