Natural Born Privacy redraws everything on your Mac as coloured dots. The real picture is in the red light only, and the green is set so that every spot is equally bright with or without a letter on it. Through red lenses the dots vanish and the text reads normally. The person next to you sees the dots.
This first build isn't notarized by Apple yet. If macOS refuses to open it, go to System Settings → Privacy & Security and click Open Anyway.
It covers every display, the menu bar and full-screen apps, and you keep clicking and typing as usual. In Dark Mode, choose Invert so that dark backgrounds become the dots. The menu also sets how dark the text is through the glasses: lighter text is harder to catch without them. If your lenses let a little green through and you see faint dots, raise Lens Leak Correction until they are gone. Blue lenses work too; green lenses get the simpler red-and-white dots of the original.
The app reads the picture of your screen, recolours it and shows the result on top. That is why it needs the Screen Recording permission. Nothing is saved, and the app makes no network connections.
This is a privacy screen, not encryption. Brightness gives nothing away, but text keeps a slightly different tint from the page, so someone who stares from close by can still make out large headings. Photos and video become hard to recognise for you too, since the glasses leave only one colour.
The trick is from Natural Born Privacy, a Chrome extension Wim den Herder published in 2016 that does this for web pages. This app does it for the whole Mac.