Download AKBlack.7z.

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

Inside the .7z archive, you will find a folder called AKBlack. Copy this folder to /_nds/akmenunext/ui on your flashcart/console SD card.

Place the SD card back into your flashcart/console, and boot AKMenu-Next.

Press the START key to open the AKMenu-Next start menu popup, then select Themes.

Find the theme under the categories and press A to set AKBlack, then press X to save, and A again to restart.