Change Filetype Icons of Systemfiles

I want to use this icon ( I made to replace the current icon ( for all my .png files. The default program for opening .png files is Preview.

To do that I need to acces the Package Contents of the Preview App and to replace the current png.icns file with my png.icns file. The problem however, is that it doesn’t allow me to do this.

I have tried turning off SIP, doing sudo mount -uw /, giving myself/admins read/write permissions. Nothing seems to allow me to delete, edit, or replace these .icns files.

Anyone have any idea what I can do? Thanks in advance!