Remove background from scrollbars when hovering on them
Summary: Current gtk scrollbars have background that doesn't look very good and removing it makes scrollbars look better/more smilar to scrollbars in other apps.
Test Plan: No visual defects in any gtk app.
Subscribers: filipf, ngraham, ndavis, plasma-devel
Differential Revision: https://phabricator.kde.org/D21639