anchor.diff:              manage = "desktop";
               start = d.start
               + optionalString (needBGCond d) ''
  -              if test -e $HOME/.background-image; then
  +              if [ -e $HOME/.background-image ]; then
                   ${pkgs.feh}/bin/feh --bg-scale $HOME/.background-image
  +              else
  +                # Use a solid black background as fallback
  +                ${pkgs.xorg.xsetroot}/bin/xsetroot -solid black
                 fi
               '';
             }) list;
  
positive.code:   config = {
      services.xserver.displayManager.session = cfg.session.list;
      environment.systemPackages =
        mkIf cfg.session.needBGPackages [ pkgs.feh ];
    };
  }
  
negative.code:   cfg = xcfg.desktopManager;
  
    # If desktop manager `d' isn't capable of setting a background and
    # the xserver is enabled, `feh' or `xsetroot' are used as a fallback.
    needBGCond = d: ! (d ? bgSupport && d.bgSupport) && xcfg.enable;
  
  in
  