Actually, it looks like there’s an even newer version on his page, here:
http://blog.nilsbyte.de/downloads/
To completely replace the broken theme (in RetroPie 2.3 or 2.4, at least), delete the /etc/emulationstation/themes/simple folder and replace it with the simple folder from the downloaded zip file. I would make a backup first, just in case.
Once it’s working, if you want to try editing it again, I would recommend making a copy of the simple folder with a different name, and making the edits there. That will effectively create a new theme that you can switch to from the start->UI menu in EmulationStation. If that breaks, you can always switch it back.