Is there a way to turn off flat UI in MacOS Sierra? 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


Open System Preferences > Accessibility > Display
Tick the checkbox "Increase contrast"




Description


  Increase contrast
  
  Increase the contrast of items on the screen (such as borders around
  buttons or boxes) without changing the contrast of the screen itself.


Source: Apple.com