Install icnsutils by entering the following commands in the terminal:
sudo apt update sudo apt install icnsutils
Description:
utilities for manipulating Mac OS icns files
icnsutils includes icns2png and png2icns, two utilies used to extract PNG images from icns files, and create icns files from PNG images.
Homepage: http://icns.sourceforge.net/
Version: 0.8.1-3.1
Section: universe/graphics