code: function(opacity) { 
          if (opacity != undefined) {
              this.opacity = opacity; 
          }
          
          if (this.div != null) {
              // for Mozilla and Safari
              this.div.style.opacity = this.opacity;
  
              // for IE
              this.div.style.filter = 'alpha(opacity=' + this.opacity*100 + ')';
          }
      }
docs: Method: setOpacity
  Sets the opacity of the popup.
  
  Parameters:
  opacity - {float} A value between 0.0 (transparent) and 1.0 (solid).