Locksmith
Locksmith is a terminal tool for changing the visual parts of macOS that Apple does not make particularly easy to manage.
It can set the login screen, desktop wallpaper, screensaver and user account image, either interactively or from the command line.
It works across current and older macOS versions, handling the different mechanisms used by each without requiring you to dig through system files yourself.
I built it for managing Mac minis and other machines in an agentic setup, where being able to see immediately which machine is which becomes surprisingly useful.