I’m on Debian with XFCE I’ve already tried modifying /etc/default/grub and /etc/lightdm/lightdm-gtk-greeter.conf The first resulted in nothing (I actually have grub timeout set to 0, so I’m not sure if I only need changing grub’s config), the bootsplash was still default The second resulted in black login bg


Pretty sure you have to rebuild the grub
update-grubafter editing the config files.There are programs to help like grub-custonizer.
Careful as messing with grub can break your system.