Ok now I see. Then you'll need to create a new symbol and set it as the map's info window default fill symbol. For example this line sets it to a transparent black color. Note that if your code doesn't already load the esri/symbols/SimpleFillSymbol and esri/Color modules you'll need to load them.
this.map.infoWindow.set("fillSymbol", new SimpleFillSymbol().setColor(new esriColor([0,0,0,.4])));
For the simple viewer template a good spot to add that code would be right after the line that adds the light class to the map's info window at approximately line 285 of main.js.
I've attached an updated main.js file that contains the updates.