if you're using debian (13), you'll see that regardless of the GRUB theme you install, there is a debian background set for the GRUB console.

turns out, you can change that by simply putting an image file in "/boot/grub", then updating grub ("sudo update-grub"). the next time you reboot and open the console or boot into debian, you'll see the image you've put.
if you wanna get rid of the console background entirely, you can use a black 1x1 image, or delete "/etc/grub.d/05_debian_theme".