Style the resize handles

GoJS
1

Is there a way to style the resize/rotate handles?

2

Yes. There are examples at:
https://gojs.net/latest/intro/tools.html#ResizingTool
https://gojs.net/latest/intro/tools.html#RotatingTool

Also you can find the built-in definitions at:
https://gojs.net/latest/extensions/Templates.js