Is it possible to set a glyphicon or custom class on a GoJS button?
One cannot apply CSS to any GraphObject.
If your icon is a PNG file, you can display it in a Picture.
If your icon is an SVG file, you can either render it in a Picture or in a Shape. Please read GoJS Pictures -- Northwoods Software or look at how you can use SVG icons: Icons GoJS Sample
I was thinking of adding Glyphicons that are included with Bootstrap. Ok, I see they are defined in a svg file, I guess I can load them then…