OK, you can fetch and install it (in arch-chroot) with commands
wget https://github.com/endeavouros-team/archive/releases/download/packages/grub2-theme-endeavouros-20220602-1-any.pkg.tar.zst
pacman -U grub2-theme-endeavouros-20220602-1-any.pkg.tar.zst
Note that you probably should edit /etc/default/grub to use the grub theme like before, and run grub-mkconfig -o /boot/grub/grub.cfg before reboot.