Make the System Settings category headers look consistent with other Kirigami lists

Summary: {F7167951}

Test Plan: Open the System Settings.

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, #vdg, plasma-devel, #plasma

Tags: #plasma

Maniphest Tasks: T10384

Differential Revision: https://phabricator.kde.org/D22953