What is the best tool to track theme customizations on a Mac OS?

I’ve got a store that has some custom coding applied to the theme, but there is zero documentation for the store and zero documentation/comments in the code. I need to track down some customizations that have been done to the theme. I have a copy of the store’s theme (current) that has the changes and a copy of the store’s them pre-changes.

I’m basically left with a situation where I need to diff the entire theme.

Can anyone recommend a good tool for this (Mac OS please)?

Never had to do a diff of this scale before.

-J