GRUB theming - background

[白い熊]

Hi Guix:

I have this nice GNU hacker background that ages ago some system installed as 
the GRUB background then. I'd like to use it as GRUB background in Guix System 
now.

I tried to set it thus, along the lines of GRUB theming instruction in the 
manual:

(grub-configuration
(theme (grub-theme
(inherit %default-theme)
(background_image '("/home/shiroikuma/0/pictures/grub/gnu-grub.jpg")))))

But it doesn't work — gives me an invalid field specifier — so
I guess “background_image” cannot be specified this way. Any other way to do it?

-- 
Best regards / 宜しく御願い致します / S pozdravem / C уважением / Z poważaniem /
Mit freundlichen Grüßen
白い熊