Where to put grub themes?

The script from vinceliuice for modding the grub to very nice themes doesnt work currently.

What directory would grub themes go in OSTree file systems?

Or should OSTree distros use systemd-boot instead? Is there a problem with dualboot?

So on my Legacy Bios Laptop the themes are stored in /boot/grub2/themes and work. I just changed the theme dir to that location in the install.sh

But where is the directory for EFI? After trying that I got an error “Coudlnt load files”. During boot and a normal fallback grub, which looks very ugly.