Hello. No problem.
Download Wordpress
Unzip into your own PC.
Find the file: wp-content/themes/thesis_16/custom/custom_functions.php
Upload custom_functions.php to your host, replacing the bad file
... now it should work again, as you are back to start.
General Advice for Coding/modifying:
Before you begin to modify and change in a working code page,
take a BACKUP (BACKUP) copy of this file you will change.
When I do this myself I will name such a backup file:
default_custom_functions.php
and put it in same directory as custom_functions.php
When things go wrong or I need to have a look into default original file
I can easy do it.
Regards 🙂