I can't comment because of rep but I would like to add to this because people now might come across it and want to pull this off. With Sierra's SIP implementation, Adam's method no longer works by default due to the inability to copy files to any CoreServices subdirectories. You would have to disable SIP first.
Before I go further you must understand that SIP is a protective measure put in place by Apple to make sure the integrity of your system is not compromised.
I will not go into how I feel about SIP as an application developer but suffice it to say that I'm not always in sync with Apple's decision making.
To do this:
Reboot your Mac.
When you hear the chime, press and hold CMD+R until you see the desktop start to appear.
From the 'Utilities' menu on the top, select 'Terminal'
Type "csrutil disable" and hit enter.
Type "reboot" and hit enter.
SIP is now disabled. I generally don't care for SIP myself but understand I'm in the minority. Also, I don't want Apple too mad at me. That said, I recommend if you are doing this to replace a system icon that you reenable SIP immediately when you are done. To do so, follow the instructions to disable but replace the word "disable" with "enable".