[GTK3] Fix treeview hover colour

Summary:
Treeviews now reflect the Qt theme more accurately when hovering items.

BUG: 414263
FIXED-IN: 5.18

Test Plan:
Before:
{F7877905}
After:
{F7877902}

Reviewers: #breeze, #vdg, ngraham

Reviewed By: #breeze, #vdg, ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26591