In: https://wiki.blender.org/wiki/Reference/Release_Notes/2.80/Python_API/UI_DESIGN
At Toolbar Icons it’s speficied how to create your icons in the .blend file. But later the exporting process is very vague. In what directory you do need to be in order to run command:
./blender.bin \
icon_file.blend --background --python ./release/datafiles/blender_icons_geom.py -- \
--output-dir=./release/datafiles/blender_icons_geom
Anyone had any luck with this?