Changing App icons to make them different (the exact opposite to my last question)

Last time, I accidentally changed my safari app icon to something I didn't like. So somehow I reverted it back yesterday. I can change all apps I download online and from the App Store but I think I did something yesterday because I can't change any of the system app icons. My MacBook doesn't support Big Sur so I wanted to change the icons to Big Sur style. I do the same process as App Store icons but system icons just don't want to change. And to say that yesterday I accidentally changed Safari to some weird picture I had somewhere. I wonder why everybody thinks Big Sur ugly, I personally didn't macOS icons until this update. Thanks :)