Scroll bar at bottom of editor window -scroller appearance

Is it possible to change the appearance of the scroller, the thing you drag with mouse to move along the tracks, at the bottom of the editor window.
I had a look in Preferences/Theme but couldn’t find a setting for this but I’m not sure of the correct terminology so may have missed it.
What I want to do increase the contrast or darken it. I use track-coloured regions.
gcc5 4.7-5-g318f385