[root@nixos:~]# head -n 40  /mnt/etc/nixos/configuration.nix
# Edit this configuration file to define what should be installed on
# your system.  Help is available in the configuration.nix(5) man page
# and in the NixOS manual (accessible by running ‘nixos-help’).

{ config, pkgs, lib, ... }:

let
  oldTarball = fetchTarball https://nixos.org/channels/nixos-18.09/nixexprs.tar.xz;
in

{
  imports =
    [ # Include the results of the hardware scan.
      ./hardware-configuration.nix
    ];

  nixpkgs.config = {
        allowUnfree = true;
        packageOverrides = pkgs: {
                old = import oldTarball {
                config = config.nixpkgs.config;
      };

                 # lxappearance bug(?) https://github.com/NixOS/nixpkgs/issues/26909
                lxappearance = pkgs.lxappearance.overrideAttrs(old: rec {
                        name = "lxappearance-0.6.2";
                        src = pkgs.fetchurl {
                        url = "mirror://sourceforge/project/lxde/LXAppearance/${name}.tar.xz";
                       sha256 = "07r0xbi6504zjnbpan7zrn7gi4j0kbsqqfpj8v2x94gr05p16qj4";
                   };
    });
    };
  };