Why should I make a backup of my theme?
Before installing any app, its highly recommended to make a backup of your existing theme.
The simple reason is making backups are a great idea incase something changes that you need to undo. Many apps (DonateMate included) will make changes to your theme, and while I only make small changes that should in no way impact your other products, it might be easier to just revert to a backup than trying to remove all the added code.
This is especially true if you decide to uninstall an app. Once the app is uninstalled, developers like myself are unable to make further changes to your shop. This is a good thing, but it also means I can't help you remove the extra code that DonateMate added.
The easiest way to do this is to go back to a backup.