@helmers also posted on this subject in his blog: https://helmers.vivaldi.net/better-scrollbars/
True. You could add something like this. This will put a one-directional incremental at each end:
This will put bidirectional incrementals on each end:
border: 1px solid black;
To get an arrow image on the incremental I guess you'd have to find/create an image with the right background color and encode it as an URL.