A few months ago, I upgraded my desktop computer from a Windows 7 to a Windows 10. It was fine, nothing was really wrong with it, it worked great. Soon I got sick of the aesthetic design of it, and wanted to downgrade again.
(I already tried the UXStyles thing and it doesn't work at all for some reason)
I read somewhere that in order to downgrade, you must've used Windows 10 for 30 days or lower. Anything above means your old Windows files have been deleted.