[Separator] Match Breeze borders' color
Summary:
Kirigami separators are currently stronger than Breeze style borders used with controls.
With this patch the difference between the two should no longer be observable.
Test Plan:
Breeze
Before:
After:
Breeze Dark
Before:
After:
Reviewers: Kirigami, VDG, ngraham
Reviewed By: Kirigami, VDG, ngraham
Subscribers: ndavis, mglb, apol, plasma-devel
Tags: Kirigami
Differential Revision: https://phabricator.kde.org/D22106