I can't find a proper way of setting gtk themes under NixOS. My usual approach is lxappearance, but after installing some gtk-themes with nix lxappearance can't find them (as there is no /usr/share/themes, but the themes aren't under .local/share/themes). In the case of fonts I can just use the fonts.fonts option in configuration.nix, but there is no option for this in the case of icons and themes. The only thing I found so far is this old config.nix with some scripts, but it is a really hacky solution, which I would rather not use.
I am using bspwm as window manager. Also, I tried installing it with configuration.nix as well as nix-env -i.