Even as admin, I cannot edit DefaultFonts.plist for Notes.app. What am I doing wrong? I want to modify DefaultFonts.plist for the Notes application. I've tried a few different ways since what worked on old OS doesn't on 10.14 Mojave:


I've navigated to the file through finder, went to Get Info, unlocked the file (via the lock icon), and tried to change permissions. 


I get the message "The operation can’t be completed because you don’t have the necessary permission."

I've tried this command:

chmod 777 /Applications/Notes.app/Contents/Resources/en.lproj/DefaultFonts.plist

I get the message: Operation note permitted

I've also tried:

sudo nano /Applications/Notes.app/Contents/Resources/en.lproj/DefaultFonts.plist

I'm able to get into the editor, but when I try to save I get message: "error writing /Applications/Notes.app/Contents/Resources/en.lproj/DefaultFonts.plist"

also, I cannot cd to /Applications/Notes.app/Contents/Resources/en.lproj. (And I cannot see Notes.app when I'm in the Applications folder.)

The application is closed while I'm doing all this.

I have admin permissions on my computer. What in the world am I doing wrong here?

UPDATE: I'm now able to edit the file thanks to @bmike. However, changing the font size values in the file doesn't seem to affect the application. I think I have the correct file. (btw, not sure if I should be writing this follow-up here or just in comments...or creating a new post. Feedback welcome.) Created new post to address this: Notes.app reverts back to default font when using bulleted list
Several of Apple's bundled applications are protected by SIP (System Integrity Protection). As a result, they cannot be modified, even with root privileges. The -lO flag for ls will show you if a file or folder has restricted status:

ls -lO /Applications/Notes.app/Contents/Resources/en.lproj/DefaultFonts.plist
-rw-r--r--  1 root  wheel  restricted,compressed 538 Aug 20  2018 /Applications/Notes.app/Contents/Resources/en.lproj/DefaultFonts.plist


If you want to proceed, you'll have to disable SIP, make the modifications, and then consider re-enabling SIP. (I strongly recommend not leaving it off, as it provides valuable security for your Mac against malware and other issues like inadvertent destruction of system critical files.)

If you don't like Apple's procedure for disabling SIP, You'll find many webpages explaining how to disable SIP. Here's one.


https://support.apple.com/en-us/HT204899


However, it's possible that altering the app directly may compromise its code-signed status. (Again, a security feature designed to stop apps from running if they've been modified.) You may need to look for another way of changing the font in Notes.