Use text color for "No preview image" text

Summary:
The text that shows up as the theme preview when no image preview is available was always just black.

That's not great for dark themes so this patch makes it use text color.

Test Plan:
Before:

After:

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25971