anchor.diff:  
             </fontconfig>
           '';
  +    in mkIf fontconfig.enable {
  +
  +      # Fontconfig 2.10 backward compatibility
  +
  +      # Bring in the default (upstream) fontconfig configuration, only for fontconfig 2.10
  +      environment.etc."fonts/fonts.conf".source =
  +        pkgs.makeFontsConf { fontconfig = pkgs.fontconfig_210; fontDirectories = config.fonts.fonts; };
  +
  +      environment.etc."fonts/conf.d/98-nixos.conf".text = nixosConf;
   
         # Versioned fontconfig > 2.10. Take shared fonts.conf from fontconfig.
         # Otherwise specify only font directories.
  
positive.code: 
          defaultFonts = {
            monospace = mkOption {
              type = types.str;
              default = "DejaVu Sans Mono";
              description = ''
                System-wide default monospace font. The default is not set if the
                option is set to <literal>""</literal>.
              '';
            };
  
            sansSerif = mkOption {
              type = types.str;
              default = "DejaVu Sans";
              description = ''
                System-wide default sans serif font. The default is not set if the
                option is set to <literal>""</literal>.
              '';
            };
  
            serif = mkOption {
              type = types.str;
              default = "DejaVu Serif";
              description = ''
                System-wide default serif font. The default is not set if the
                option is set to <literal>""</literal>.
              '';
            };
          };
  
negative.code:   config =
      let fontconfig = config.fonts.fontconfig;
          fcBool = x: "<bool>" + (if x then "true" else "false") + "</bool>";
          nixosConf = ''
            <?xml version='1.0'?>
            <!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
            <fontconfig>
  