Notice those color dots to the top left corner of any MacOS window? I do from time to time. I never would have thought about changing them, until Apple introduced me to the feature a couple editions ago. I started going thru a 'dark mode' phase around the same time. In the process, I was finding new ways to de-color my 5K display. Yes, I know it sounds a bit odd, but hear me out.
I work in the media production business, and I do a bit of software engineering. Both tasks tend to offer tools (such as video editors or IDEs) with a dark interface. It helps you focus more on what you are creating, and reduce the distraction from the tool being used. Finally, Apple and Microsoft started incorporating this design into their operating systems. In Apple's case, the features trickled in over a couple iterations of their operating system.
I regularly use two laptops, both of them MacBook Pros, varying release years. At one point, I changed one of them to use a gray color or shade everywhere I could. This was probably back during MacOS 10.13.
On my other laptop, I decided to return to colors for those window control buttons, at some point past my initial infatuation with removing all colors. So, what's the problem? Well, recently a coworker asked me about changing their interface back to using colors. I said it was easy to change, and started to explain where to go. They quickly stopped me and said it was not there, under the General settings any longer. I scoffed and turned to my MacBook and after searching for a while, I came to the conclusion that the ability to change that setting thru the GUI is gone. I suspect there is some 'convenient' terminal command I could type in (from memory of course), which would restore my computers to each have the same setting. I will be searching for that command, while I wait for Apple to restore the control in the GUI.