I believe there's more manual labor involved in making a Big Sur theme for the masses that can shift the white-balance a little bit.
On the topic on the actual theme of Big Sur, theres lot to be done. For instance, PaintCan, a MacForge plugin, allows for "hot-swapping" system theme .car files. If you use the ThemeEngine version released by Jeremy Legendre on GitHub, then you can then make use of my Xcode projects to make yourself the MICA files needed for greying up the light theme in Big Sur.
With a little luck I can provide some Aqua traffic lights tomorrow too.
PaintCan on GitHub or download within the MacForge application
ThemeEngine from Jeremy Legendre forked from Alex Zielenski who is the original author.
Download my Xcode projects here and see if you can make those MICA files for Big Sur
(the above is a link to my own dA account that requires login to download. That is something Deviantart set up and not me. You can use that link if you want to or I have attached the rather small archive here. You can still read the text on the above link)
I did actually have a theme ready to go for Big Sur that I of course have deleted because I got so tired of all the hoops of theming Big Sur - the whole world was against me at one point. The links in the following thread does not work and you'll notice that me writing a comment to an old friend explaining. This particular theme was frustrating, not to make, but to have the gradient not mess with certain aspects of Big Sur. The Calendar app for one, Notes and Messages too.
I will see what can be done to that mess later on tomorrow. My eye surgery should clear up in the next 14 days and I ought to proceed full tilt theming
Requires PaintCan plugin for MacForge - had to quickly add that. Also, in addition to SystemIntegrityProtection to be disabled/off - you'll also have to disable LibraryValidation All this hacking and cracking macOS Big Sur is getting long in the tooth. I doubt any more work will be done to this...
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