Honestly, I am not entirely sure without seeing all of it together (and maybe not sure then either). I just did a little training once where I was 'taught' how to change that particular line of code in order to change the color of the highlighted box around a selected feature.
I was modifying this code in the Esri sandbox:
ArcGIS API for JavaScript Sandbox
Under
require([
you have to add
"esri/Color",
Under
], function(:
you have to add
Color,
Then under
var map = response.map;
is where you add this line (with whatever color you choose)
map.infoWindow.markerSymbol.outline.color = new Color("purple");
So, the bottom line is, if you see js code that is similar to the code in the sandbox link I posted above, that is where you would modify the these elements. I am not sure if the widget.js code would have that kind of code. I hope this helps!