Download Mario 3 Simple.7z.

Open/Extract the .7z archive. You can use 7-Zip to do this.

Inside the 7z archive, you will find a folder called Mario 3 Simple. Copy this folder to /__rpg/ui (or /__aio/ui for AKAIO) on your flashcart’s SD card.

Place the SD card back into your cart, and boot into the cart.

Press the START key to open the AKMenu/WoodR4 start menu popup, then select Settings.

Set the theme to Mario 3 Simple, then press A to save, and A again to restart.