Hi!
I think I managed to solve the issue with the escaping characters. I’m attaching the fixed version of the plugin. You can safely deactivate and delete the previous version, the created instances and options will remain.
I’ve tested with multiple quotes, special characters, backslashes, and it outputted exactly the input, I hope it will work for you as well.
You may need to empty the custom css field first, save it a couple of times and then enter your custom code, just to make sure it’s refreshed in the database.