Summary:
Generally, left sidebars in Kirigami-style apps (e.g. System Settings) use the view
background color. Let's do the same in Discover when it's displaying the Desktop view.

Need to bump the Kirigami version to have access to `Theme.colorSet`

Test Plan:
Sidebar is now a pleasing off-white, same as System Settings
{F6824729}

Reviewers: #vdg, #discover_software_store, apol

Reviewed By: #discover_software_store, apol

Subscribers: mart, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21229