It was already a feature in Windows 1.0 to let you customise the colours of the UI elements, and it lasted (with gradual improvements) until Windows 8 where it was removed completely.
I still miss the Windows Classic theme. It was simplistic enough while being streamlined enough for most workflows, now I have to hover icons to find specific windows in an application group or deal with the internet-connected Search/Run box.
It wasn't perfect, but it worked better for me, at the very least.
https://github.com/dremin/RetroBar gives at least the classic taskbar back. I have been using it on windows 10 for a while now. It is much better experience.