Summary:
This patch is shamelessly stolen from Marco Martin's work in KInfoCenter (23fa369831709f43a3100476b00ced10a1123f7f) and does the same thing for System Settings' tooltips.

Now Monochrome icons are displayed properly.

BUG: F6698413
FIXED-IN: 5.15.4

Test Plan: {F6698413, size=full}

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19834