Themed scroll bars

[Holmgren825]

Firstly, thanks for a wonderful plugin! It really improves my workflow in joplin.

It would be great if the scroll bars in the board pane followed the theme of joplin. Current if one uses the dark theme, the scroll bars of a note are dark and very un-intrusive. In the board however, they are of a light color (grey) and rather wide.
Example:

My knowledge of JS/React is very limited, otherwise I would be happy to contribute.