GNU artwork configuration from SLIM in GDM

[白い熊 <help-guix_gnu.org@sumou.com>]

Hi Guix:

I've long had this nice GNU blue artwork configured as background in SLIM login - thanks to help here thus:

(define %old-artwork
(origin
(method git-fetch)
(uri (git-reference
(url "git://git.savannah.gnu.org/guix/guix-artwork.git")
(commit "94e6111"))) ;pre-GuixSD branding
(sha256
(base32
"0aax85mygpq4lbyn8sriafikyg54yss3lisin6pwkvlvp23a3v1j"))))
(define %old-slim-theme
#~(string-append #$%old-artwork "/slim"))
(define %old-slim-theme-name
"0.8")

and in the bootloader section:

(service slim-service-type (slim-configuration (auto-login-session (file-append stumpwm "/run/current-system/profile/bin/stumpwm"))
(default-user "shiroikuma")
(theme %old-slim-theme)
(theme-name %old-slim-theme-name)))
%base-services))

The problem with this is I can only use it with %base-services

If I try to substitute the last line, in line with the manual, with:

(remove (lambda (service)
(eq? (service-kind service) slim-service))
%desktop-services)))

it won't configure — says xorg-server provided more once.

In principle, I'd like to use gdm - so I'd like to ask:

How to configure gdm with the above artwork?

or

How to use the above config, but with %desktop-services so full desktop services are provided upon login?

Many thanks. :@)

-- 
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白い熊