Some missing icons

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 NixOS - NixOS 21.05 manual 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.

Any pointers?