nwg-drawer expects it's swg icons in directory at path $XDG_DATA_DIRS/nwg-drawer/img (which resolves to /usr/local/share/nwg-drawer/img) It doesn't show any icon if there's no file there Or shows blank icon. Reproducible: Always
see also https://github.com/nwg-piotr/nwg-drawer/issues/122 as alternative