Use correct KPageDialog face style for settings window

Summary:
This was always a bug, but masked by the fact that the old sidebar style looked okay
without it. The new one requires that this be set correctly, so let's do it.

Test Plan:

Subscribers: Spectacle

Tags: Spectacle

Maniphest Tasks: T11279

Differential Revision: https://phabricator.kde.org/D22745