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.

Reviewers: #vdg, #breeze, ndavis, ngraham

Reviewed By: #vdg, #breeze, ndavis, ngraham

Subscribers: filipf, ngraham, ndavis, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21639