I recently "upgraded" to macOS Sierra (10.12). I understand that many people have opinions on efficient and aesthetic UIs, but I personally hate (and that is the correct word) a flat UI. If you like it, great… but is there a way for those of us who don't like it turn it off?
I'm guessing the answer is no – and I've gone through every preference and option I can think might have any bearing. My Google search didn't turn up anything, either… but there's always a chance that I'm missing something.
If it helps an answer, feel free to get technical: I've been using Apple since the mid-80s, and computers before that; I've been working as a Software Engineer for decades (I'd prefer not to admit how long), and the command line doesn't bother me. (Yes, yes, I understand that I've asked for a non-flat UI and use the command line. It's not really hypocrisy… technically.)
Tl; DR Is there an option/method to turn off the flat UI in MacOS Sierra (10.12)?
No, you can't revert back to the UI of previous versions of OS X with Apple's tools.
You can, however, make the UI elements stand out more.
macOS Sierra: Increase the UI's contrast