From the site:
Replacicon makes it easy to find and replace app icons on the Mac. It scans online sources containing thousands of replacement app icons for each of your installed macOS apps. In just a click, you can change your app icons so they better match the rest of the system, or your own tastes. […]
Replacicon even watches for app updates in the background so your customized icons are not lost across app updates.
It only works on Big Sur and I don’t have any machines that have been updated to it yet. But I still bought this app. It seems like a great piece of software.