Add a subtle background to editor scrollbars

This makes it possible to see whether a scrollbar grabber is at the top
or at the bottom of a scrollbar. Also, if a scrollable area is very
large, this makes it easier to notice that the area can be scrolled
(since the grabber is proportionally very small).

The scrollbar grabbers were also made thicker and slightly more opaque
for better visibility, especially in peripheral vision.