To use a custom icon, first enable the Custom Icon setting (duh) in the Window Utils tab.

Icon files

• To add a custom icon, you need the following icon files:
  • icon_16x16.png
  • icon_32x32.png
  • icon_48x48.png
  • icon_128x128.png
  • icon_256x256.png
  • minecraft.icns (For all your macOS users)

Now, put these icon files in this location:

.minecraft
└── config
    └── mutils
        └── icons

Now, you should be all set! As always, if you still have any questions or if something goes wrong, you can ask for help on our Discord server.