How to style gojs canvas? I want to add scrollbar into it

GoJS
1

where to apply this style to change the default style of the canvas

2

Whether or not there is a scrollbar or two depends on the relative sizes and positions of the Diagram.documentBounds and the Diagram.viewportBounds. The scrollbars are provided and styled automatically by the browser/platform.
http://gojs.net/latest/intro/viewport.html