There are some users who want to use the good-old Windows Classic theme on their Windows 10/8 installations. Here are two options that you have:

1] Download this theme from DeviantArt. It is close to the original theme, but you may face issues rendering correctly, when opening Universal apps. Do read the instructions on how to install it.

2] Download another theme from here.

Create a system restore point and use it at your own risk. The downloaded files may make your antivirus give out a warning, but its developers say its a false positive.