How to change the initial login screen background in macOS Ventura?

Question

After upgrading my macOS to Ventura, the initial login screen background, before selecting a user, is set to Chroma Red, and I can't figure out how to change it. When a user is selected, the default screen background of Ventura is shown, which can be changed by adding an image to the /Library/Caches/Desktop Pictures folder. The question is, how to change the initial login background?

Answer 1

It is possible, but there are prerequisites that are not advised for most users:

1. FileVault disabled
2. SIP (System Integrity Protection) disabled
3. SSV (Signed System Volume) disabled

If you do these things, and leave it that way, then yes, it can be done.

The image in question is /System/Library/Desktop Pictures/Ventura Graphic.heic.

If the prerequisites are completed (from Terminal window entered after Recovery reboot, csrutil disable, and then csrutil authenticated-root disable, and reboot), you can replace the above with an equally-sized (6016x6016) HEIC file (using Preview will do, just square/crop it up and Tools->Adjust Size options to get size right). The filename must be the same (copy over the original with replacement image file of same name).

But in order to make this change, the /System volume must be mounted for edit, because /System is read-only. Create a mount point (mkdir -p -m777 ~/sysmnt) and mount /System there (e.g., sudo mount -o no browse -t apfs /dev/disk3s1 ~/sysmnt); you will need to know which disk slice is your system volume - use diskutil list to find the correct disk slice for mount. Enter the mounted volume (cd ~/sysmnt), and make edits in /System paths relative to this mount point.

When edits are done, 'bless' the modified /System volume, so it is persistent at next reboot (for Intel box use: sudo bless --folder ~/sysmnt/System/Library/CoreServices --bootefi --create-snapshot , for Apple silicon [M1/M2] use: sudo bless --mount ~/sysmnt/System/Library/CoreServices --setBoot --create-snapshot). reboot.

The default desktop background can likewise be changed (6016x6016 JPEG file rather than HEIC - can use File->Export from Preview of same image from above) at /System/Library/PrivateFrameworks/SystemDesktopAppearance.framework/Versions/A/Resources/DefaultBackground.jpg .

These changes will remain, and function as expected, until the security features disabled as prereqs are restored - doing so will wipe out these changes, and the Apple defaults will be restored. As such, it can serve as an exercise to prove the concept - it does work - but not recommended to leave it this way.

Apple should just allow these changes from System Settings, and avoid this nonsense.