OK, maybe this would be easier if you're changing a lot of icons, and it will also save you having to overwrite any of the original icons

1) Copy your opt/usr/share/icons/hicolor folder to your PC's desktop
2) Change all the icons you want
3) Rename the hicolor folder to something else
4) Put the newly named folder in opt/usr/share/icons/
5) Select that folder (it will show as an icon theme) in theme customizer

Kind Regards,
D4rK