Change background between grub boot and login screen

[ping-wu]

For changing the grub boot background, I can simply copy a png file to the /boot/grub directory, or use the grub-customizer package. Wrt the login window, I can run the focalgdm3.sh bash script. Pretty straightforward.

How do I change the background in between？

[deadflowr]

Learn about plymouth themes.
https://wiki.ubuntu.com/Plymouth
https://askubuntu.com/questions/1046370/how-to-change-boot-splash-screen-in-18-04
https://www.gnome-look.org/browse/cat/108/order/latest/
That should give you a start pointing in the direction you want.