When using a different background color, use highlightedText as text color

Summary: Make sure we are using a color designed to contrast with a background color.

Test Plan: Looks better on breeze, breeze dark and obsidian

Reviewers: Kirigami, mart, ngraham

Reviewed By: Kirigami, ngraham

Subscribers: abetts, colomar, ngraham, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D13334