Use correct sidebar back button press color

Summary:
The system settings currently use a wrong sidebar back button press color, which can look very bad on some color schemes (for example Breeze Light), this fixes it.
Before:

After:

Test Plan: Open the system settings, click on any category and then press the back button.

Reviewers: Plasma, VDG, davidedmundson, ngraham

Reviewed By: Plasma, VDG, davidedmundson, ngraham

Subscribers: ngraham, plasma-devel, VDG, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18559