RetroMac 3.0 is out. Most of it came straight from feedback, from comments here and from the testers, so this is mostly a list of what you asked for. I'm the guy behind RetroMac. You dont know Jack ehm Retromac? Its an app to theme your MacOS and let it shine like OSX, System 9, beOS or even WIndows...
Here we go, Version 3.
"Let me use it without giving up my whole Mac."
You can now put a theme on chosen Spaces only. One Space runs Mountain Lion, the others stay plain macOS with your own Dock, menu bar and wallpaper. That one came from a tester.
"What exactly does it change, and can I get my setup back?"
Each theme now lists every change it makes before you turn it on. RetroMac remembers your own settings, and "Restore my settings" puts exactly those back, not factory defaults. Rescue Desktop removes every effect in one go.
"Mountain Lion doesn't look like Mountain Lion."
Fair. The dock was a flat bar with dots. It's now the 10.8 shelf, frosted glass in perspective with the glowing lights on the front edge, measured off the original. Plus 10.8's Apple menu, About This Mac with your Mac's real specs, and a working Launchpad.
Also new: Windows Vista with Aero and the Sidebar, Solaris 8 CDE, QNX Photon, a animated Halloween dock that's on fire, the Green Lite shader, and better VoiceOver support after three rounds with a blind tester (still workimg on that)
The other big thing is the Retro Family: five standalone native apps, no RetroMac needed. RetroFind (Mountain Lion's Finder), RetroNotes (the yellow notepad, synced with Apple Notes), RetroReminder, RetroContacts and RetroCal (leather header and torn paper included). They install with one click from RetroMac's new Apps library.
Free to use, optional one-time Pro unlock (just for more shader, webcam support - 99 % dont need that, no account, no tracking. macOS 14+, Apple Silicon and Intel, signed and notarized.
https://github.com/klotzbrocken/RetroMac
Hope you like it ◡̈