anchor.diff:        active-opacity   = ${cfg.activeOpacity};
         inactive-opacity = ${cfg.inactiveOpacity};
         menu-opacity     = ${cfg.menuOpacity};
  -      
  +
  +      ${opacityRules}
  +
         # other options
         backend = ${toJSON cfg.backend};
         vsync = ${toJSON cfg.vSync};
  
positive.code: 
    cfg = config.services.compton;
  
    configFile = pkgs.writeText "compton.conf"
      (optionalString cfg.fade ''
        # fading
        fading = true;
  
negative.code:       example = "0.8";
        description = ''
          Opacity of dropdown and popup menu.
        '';
      };
  
      opacityRules = mkOption {
        type = types.listOf types.str;
        default = [];
        description = ''
          Opacity rules to be handled by compton.
        '';
      };
  
  