You do not need this addon to customize icons!!! ( IconThemes )

[OleksandrMtemp1]

You od not need .rcc or .qrc files. I have Windows 10 and FreeCAD 1.2.0dev and I just paste link to Tools -> Edit Parameters -> Preserences (folder) -> Bitmaps (folder) and I just add New String Item with path to .svg files.

- parameter mame must be like: CustomPath0, CustomPath1, CustomPath2...
- files must me .svg, not .png or other file type

I think, names of files mush match names of files in FreeCAD (just find all .svg on github. And after that, just restart FreeCAD