Thanks for the patch!
Comments below:

>      (arguments
> -     `(#:modules ((guix build utils))
> -       #:builder
> -       (begin
> -         (use-modules (guix build utils))
> -         (copy-recursively (assoc-ref %build-inputs "source") "icons")
> -         (substitute* "icons/Delft/index.theme"
> -           (("gnome") "Adwaita"))
> -         (delete-file "icons/README.md")
> -         (delete-file "icons/LICENSE")
> -         (delete-file "icons/logo.jpg")
> -         (copy-recursively "icons" (string-append %output "/share/icons")))))
> +     `(#:install-plan
> +       '(,@(append-map (lambda (file)
> +                         `((,file "share/icons/")
> +                           (,(string-append file "-Dark") "share/icons/")
> +                           (,(string-append file "-Darker") "share/icons/")
> +                           (,(string-append file "-Darkest") "share/icons/")))
> +                       '("Delft" "Delft-Amber" "Delft-Aqua" "Delft-Blue"
> +                         "Delft-Gray" "Delft-Green" "Delft-Mint" "Delft-Purple"
> +                         "Delft-Red" "Delft-Teal")))))

Interesting use of install-plan, but wouldn't it be to copy everything
and simply exclude the previous delete files?  E.g. (untested):

#:install-plan
`(("." "./" #:exclude ("README.md" "LICENSE" "logo.jpg")))

-- 
Pierre Neidhardt
https://ambrevar.xyz/