grub-mkstandalone creates an image designed to be held in memory. grub-mkstandalone comes with a --themes flag that could be used to specify a theme, but depending on the modules installed, you may encounter this error like I did.
grub-mkstandalone: error: core image is too big (0x1d71b3 > 0x78000).
This means that the image generated by grub-mkstandalone was too large.