Don't use hardcoded background color

Summary:
Fixes the hardcoded SDDM theme preview background color

Test Plan: Open the SDDM KCM

Reviewers: VDG, Breeze, davidedmundson, ngraham

Reviewed By: VDG, Breeze, davidedmundson, ngraham

Subscribers: ngraham, davidedmundson, Breeze, plasma-devel, VDG

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18016