You'll be booted into macOS Recovery. In the Menu Bar in Utilities, open terminal and type in "csrutil disable" without the quotes. Restart and boot normally in macOS
Open Terminal and do "sudo mount -uw /" without the quotes. Then enter in "killall Finder" without the quotes. Close the terminal.
Edit your System Icons. Once done, reboot back into macOS Recovery and open the terminal and type in "csrutil enable"