Don't use hardcoded background color

Summary:
Fixes the hardcoded SDDM theme preview background color
{F6530004}
{F6530005}

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