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).
{F6584513}

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