I’m using i3 as my window manager. Some of the tray icons are missing, and for instance Transmission doesn’t have any icons in it. What is peculiar is that I have almost the same configuration on my laptop, and there the icons work.
I don’t really know anything about window managers and icon themes, but I have for instance
tango-icon-theme in my
environment.systemPackages. The Nixos manual isn’t at all helpful. In https://nixos.org/nixos/manual/#sec-x11-gtk-and-qt-themes the user is instructed to install themes in
systemPackages, and the section refers to nonexistent package
qt5.qtbase.gtk. And I don’t really know if this has anything to do with themes, really.