So our ISOLINUX boot screen (what pops up for BIOS) looks nice. Plain old GRUB screen for EFI. It seems that no flavors are really doing anything with theming on their images and I don't know why. I'm especially irked as to why MATE isn't using it as they have grub2-themes-ubuntu-mate. This needs some further investigation but assuming there are no complications, we should JFDI.
I booted the Cosmic ISO in EFI mode and didn't see it. Perhaps they only use it on the installed system, since it's GRUB that boots all the installs.
You know I think a lot of people are not aware of the fact that there are two different bootloaders being used depending on the firmware interface used. That may simply be an oversight.