This is a short tutorial for those who desire to make you own icons (either completelly new, or additional to the present ones).
All the neccessary materials could be found here[drive.google.com]. You would still need to get an original (uncolored) icons from somewhere else (preferably - from the mod you coloring them for).
This file is used to make 52x52 .png colored icon backgrounds. If you need a new icon background, simply pick any color desired in a color picker pallete and fill the colored square on the second layer with yours and then save the file as a .png.
Those are only used to make lower-resolution backrounds (simply by image resizing tool).
To make your unit category icons, you would need .png files of resolutions 22x22 (regular-sized) and 44x44 (double-sized) of both - the backgrounds and the icon symbolics itself.
Those are used to make icons. Simply place your icon over those backgrounds and the work is done. Although, if you want for the icons to fit backgrounds properly, I recommend you to resize them (your icon symbolics, not the backgrounds) to the 21x21 and 42x42 sizes accordingly. After that you would only need to center them on the background and save new unit category icon by it's previous name and add to the other icons in the mod.