Summary: This patch makes the Settings page visually consistent with the Updates page. Now that both have (with the Breeze color scheme) a gray background, with white list items than span the full width. Button placement is also improved, thanks to a recent Kirigami patch that automatically moves it in a bit when a scrollbar is visible.

Test Plan:
Here's what the {nav Updates} page whose style we want to match currently looks like:
{F6172218}

Settings page, before:
{F6172219}

Settings page, after:
{F6172220}

Reviewers: apol, #discover_software_store

Reviewed By: apol, #discover_software_store

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14557