[TabBar] Use window color instead of button color
Summary:
If some color scheme (not Breeze) doesn't use the same button and window colors the tab sticks out.
BUG: 413311
FIXED-IN: 5.65
Test Plan:
Before:
After:
Reviewers: Plasma, VDG, ngraham
Reviewed By: VDG, ngraham
Subscribers: davidre, broulik, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D25581