well the tl;dr is download the skin files here
the three “animationAtlas” PNGs and the textColors.txt
https://sticknodes.com/stuff/skin_dark/

edit the PNG files as you want in the graphics app of your choice, carefully replacing the individual sections of images, colors, etc

save them and put them into a new folder you make:
/skin/HD/(PNG files go here)
/skin/textColors.txt

choose “custom skin” in the app and it’ll use those files