I recently found out about it by checking different themes to see if I still have them from a previous Windows Installation. Now, I am sharing this if you upgraded to Windows 10 without doing a clean install, or it could just be there by default as well!
Go to Settings > Personalize (Or right click the desktop > Personalize)
Go to Themes > Theme Settings
Scroll Down to High Contrast Themes.
Click the one entitled "classic"
There you go, you have classic theme on Windows 10!
Tell me if it does not work or what you think about it! Huge Shout out to MajorSky17 about this as we all know he loves the Classic theme!