Update ThemingController.php
Update ThemingController.php

Improved translation string.
Reported at Transifex. See https://www.transifex.com/nextcloud/nextcloud/translate/#de/$/121491256?issue=yes
nextcloud/server,andreas-p/nextcloud-server,andreas-p/nextcloud-server,nextcloud/server,andreas-p/nextcloud-server,nextcloud/server,andreas-p/nextcloud-server,andreas-p/nextcloud-server,nextcloud/server