Restore proper background color on list and sources pages
Summary:
Kirigami changed the defaults in 49856625cc4fb7136099ce69bfb92d99a5bca480, so we have to
adapt in Discover.
CCBUG: 404827
Test Plan:
Reviewers: apol, Discover Software Store
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D19712