* GTK themes
this should work:

#+BEGIN_SRC nix
      # https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/x11/desktop-managers/gnome3.nix
      xserver.displayManager.session = [
        {
          manage = "window";
          name = "awesome";
          start = ''
            # Set GTK_DATA_PREFIX so that GTK+ can find the themes
            export GTK_DATA_PREFIX=${config.system.path}
            # Find theme engines
            export GTK_PATH=${config.system.path}/lib/gtk-3.0:${config.system.path}/lib/gtk-2.0
            # Find the mouse
            export XCURSOR_PATH=~/.icons:${config.system.path}/share/icons
            # Update user dirs as described in http://freedesktop.org/wiki/Software/xdg-user-dirs/
            ${pkgs.xdg-user-dirs}/bin/xdg-user-dirs-update
            # Find the mouse
            export XCURSOR_PATH=~/.icons:${config.system.path}/share/icons
            ${pkgs.awesome}/bin/awesome&
            waitPID=$!
          '';
        }
      ];

        environment = {

          # Share needed for themes and backgrounds
          pathsToLink = [ "/include" "/share"];
       };
#+END_SRC