Fix incorrect colours in the KNS Quick messagebox
Summary:
Explicitly set the colours used in the messagebox' content item,
as the theme does not propagate to the simpler items, only to
those based on top of QtQuick Controls.
BUG:421270
Test Plan: Ensure you are using a dark colour scheme. Provoke an error dialog in any KNewStuff Quick dialog, and see that this patch makes the error message colour turn from a near-invisible text on a dark background (as shown in https://bugs.kde.org/show_bug.cgi?id=421270) to a visible text on a dark background.
Reviewers: Frameworks, KNewStuff, Plasma, bugseforuns, ngraham
Reviewed By: ngraham
Subscribers: ngraham, kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D29678