Have a look at this thread:
They're discussing/testing how to modify windows binary files to "get back" to classic interface by "unusual" methods, rather than just turning colors into gray!
But it appears to be very complex due to totally different structure of explorer, windows manager and whatelse in Windows 10 w.r.t. previous versions.
Note: be sure to create a restore point before running ClassicTheme.exe for testing! It can screw up whole interface!
For the start menu, things are easier thanks to ClassicShell: http://www.classicshell.net/gallery/
Additionally, try googling for:
Classic AE by Saarineames (download from deviantart)
Aero Lite Theme (among other things, make windows border larger than one pixel, for better resize and better visibility)
You can also play (carefully) with registry keys:
- HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\DWM (you can add AccentColorInactive key here, or edit existing keys)
Ultimate Windows Tweaker is also an amazing tool: