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1. Create a folder with the original (unmodified) theme.

2. Within your command line, enter that folder and type git init (Git will now record the current state of that theme)

Step 2.5: Type git add . && git commit -m 'Initial commit.'

3. Copy the files of the modified theme into that folder, overwriting the original files. (Don't worry, nothing will get lost, as Git knows what the original files were)

4. From within that folder, run git diff to get a list of the changes the modified theme did.

You can do step 3 with the command lines tools, with your Finder, or whatever else you use to copy files.