Comment 4 for bug 1796183

The package name is network-manager-gnome, but it listed under network-manager-applet here on launchpad.

It may be necessary to switch to another icon theme and back after adding those four new icon files. Also, a restart of the panel and nm-applet may be required to see any change.

Alternatively, install xubuntu-artwork from this PPA:

https://launchpad.net/~xubuntu-dev/+archive/ubuntu/ppa