D8481: Adjusted scrollbars to fit the Qt theme

[Sebastian Krzyszkowiak]

dos added reviewers: Breeze, Plasma.

REPOSITORY
  R98 Breeze for Gtk

REVISION DETAIL
  https://phabricator.kde.org/D8481

To: dos, #breeze, #plasma
Cc: plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, jensreuterberg, 
abetts, sebas, apol, mart