I made a sys-net-test, with it own template that had the icons installed.
My theme has 16,22,24,32,64,256 top level folders, I just renamed them one by one and restated sys-net-test, until it didn’t have a network icon.
Then I know what icon size was used, then it just repeated for the sub folders for that icon size, until I had the exact folder that was used.
This only changes the icon from the appVMs, it doesn’t change the icons for dom0. If you want dom0 to match the appVM icons, then you need to install the same icon theme in dom0.
You can find the Whonix icons in/usr/share/sdwdata-gui/icons, in the Whonix template.