I’m in doubt about how to style scrollbars in specific divs.
This may not be specific to Twine or Sugarcube, but someone should be able to assist. Meanwhile, see if this is any help:
Note that webkit scrollbars are not available in all browsers, for example Firefox and Edge.
If you want to be sure that your custom scrollbars will be shown in all browsers, then you’ll need to use something like Perfect-Scrollbar instead.