@aalexandrion: Hello there,
First of all you will need to create a child theme.
You can use our plugin Generate Child Theme to create child theme.
Install and Activate the plugin
Then,
Go to => Dashboard => Generate Child Theme => Select Parent Theme as Personal Trainer and fill in remaining fields and Click on Generate.
This will create and activate the child theme.
Now,
Go to => Child Theme Folder => functions.php and add the code in this link.
Let me know if this works out!
Kind Regards,
Skandha