How to Modify button properties.
For example: Shape, size, fill colors. etc.
Buttons are defined at https://gojs.net/latest/extensions/Buttons.js. Look at the definition(s) of the button(s) that you want to modify, and determine the properties that you want to set. Then you can set them on your button instance.
See for example how the “TreeExpanderButton” has been modified in the TreeView sample: GoJS Tree View
Note how you can set or bind properties on child objects if they have name
s:
$("TreeExpanderButton", { "ButtonBorder.fill": "lightyellow" })