Okay, I was finally able to take a look at it is not a bug with the plugin but with the 4.1.19 version itself. Because of a bug some hooks are not loading/working as expected.
Download the "system/Theme/Theme.php" patch from the release notes page and upload it to your server, after that the hook (and any others) will start working properly:
While you're at it you might want to install the other 4 patched files too, thought I think IPS is going to release a new 4.1.19.1 version soon to include all of those.