Update Breeze Look and Feel theme previews

Summary:
Update and improve some old Breeze Look and Feel theme previews (the Breeze Dark previews will be updated in a different revision, because they are in a different repo).

Test Plan: Open the Look and Feel KCM and the Splash Screen KCM.

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: abetts, ngraham, rooty, VDG, plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18681