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