Hi! I'm the developer of this. It is open source, as the code is publicly available on archive.org. The secure boot disabling and efi changes are needed to change the boot logo to the Windows 7 starting screen via HackBGRT (it's not LogoFAIL vulnerable). Defender is disabled so that it doesn't delete stuff, and after it's run, defender is re-enabled. The defender disabling was only a temporary measure and v2 won't have it. The administrator/trustedinstaller permissions are needed to replace many files in the Windows folder, such as icons, built-in apps, sounds, tray applets, themes, menus, cursors, and more. Everything will be improved in v2, and options to pick what you want it to do and what to skip will be available. If you'd like I can give a line-by-line explanation of the code if you are still suspicious. The reason there's only a few results is that I've only released it recently, and it takes a long time to get noticed by everyone.

TLDR, a Windows 7 theme is more complicated than you'd expect.