The custom scrollbar should use the pointer cursor rather than the move (grabber) cursor. It may also be possible to use CSS to change the look of the scrollbar rather than inventing a custom one, but I would imagine there is some functional reason for using a custom one here other than looks.
Cursor before:

Cursor after:

It's better UX to have scrollbar replacements retain the same pointer as other scrollbars; else, it ruins the illusion and feels bad.