The background color option is self-explanatory.
The mode is either `normal` or `stretch`, they are as defined by GRUB,
where normal will put the image in the top-left corner of the menu, and
stretch is the default, where it stretches the image without
consideration for the aspect ratio.
* https://www.gnu.org/software/grub/manual/grub/grub.html#background_005fimage