白い熊,

白い熊 写道：
>> (grub-theme
>> (inherit %default-theme)
>> (images
>> (list (grub-image
>> (aspect-ratio 4/3) ; or 16/9, &c., to match…
>> (file (local-file
>> "/home/shiroikuma/0/pictures/grub/gnu-grub.jpg"))))))
>
> This unfortunately also gives me “invalid field specifier”.

OK, can you be more specific?  ‘Invalid field specifier’ on its 
own doesn't tell us anything.

I pasted my previous example into my own system.scm and it built 
fine:

(operating-system
 …
 (bootloader (bootloader-configuration
              (bootloader grub-efi-bootloader)
              (target "/boot/efi")
              (theme (grub-theme
                      ;; This probably makes little sense in 
                      practice,
                      ;; unless your image suits the default 
                      colours:
                      (inherit %default-theme)
                      (images (list (grub-image
                                     (aspect-ratio 4/3)
                                     (file (local-file 
                                     "/home/…")))))))
              (timeout (seconds 1)))
 …)

What does your operating-system's (bootloader …) field look like, 
and what did Guix actually say?

Kind regards,

T G-R