virt-manager on my NixOS system was missing icons on most of its buttons, making the application handling somewhat difficult. A little stupid annoying problem, apparently of category 3, has wasted several hours of my life.
I reported it as a NixOS bug. NixOS people responded quickly and were helpful so after a couple of days I’ve found out the problem can be remedied by installing
humanity-icon-theme package. Hopefully
virt-manager NixOS package gets fixed so that users won’t have to deal with this problem again.