Properly use QCLP to get the theme name
Summary:
Instead of guessing it will be the first argument, which leads to very
weird behavior.
Test Plan:
Checked the kcm still works
Played around with it
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: davidedmundson, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D8993