Background color for sidebar headers

Summary:
kcm modules have backgroundColor as main background color.
The new systemsettings sidebar design has viewBackgroundColor as main background color.
Through this, the kcm modules look a little misplaced to me.

With this patch, the sidebar's headers have backgroundColor as their background color, matching the background
color of kcm modules. Thus, kcm modules look a little better embedded.

This might conflict other's tastes or even conflict VDG guidelines I am not aware of. So let me know what you
think.

Reviewers: Plasma: Workspaces, mart

Reviewed By: Plasma: Workspaces, mart

Subscribers: broulik, ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8277