Bug#1105055: desktop-base, gdm3: Debian logo at gdm login prompt has very poor contrast

[Simon McVittie]

Package: desktop-base,gdm3
Severity: important
Tags: trixie sid
Control: found -1 desktop-base/13.0.1
Control: found -1 gdm3/48.0-1
X-Debbugs-Cc: debian-gtk-gnome at lists.debian.org

(I think this is really RC since it's a high-visibility issue with our 
default desktop's branding, but filing as non-RC for now because I don't 
want to prevent the new desktop-base from migrating.)

To reproduce
============

Install trixie with GNOME and gdm3, then add sid as an apt source and 
upgrade desktop-base (only) to the version proposed for trixie. Reboot 
and wait for the gdm3 login prompt ("greeter" in gdm jargon).

I originally saw this on my mixed unstable/experimental laptop, and 
reproduced it on an existing trixie VM for this demonstration, but I'm 
confident that this would be reproducible with a fresh installation.

Expected result
===============

gdm3 shows a dark grey background. This is part of GNOME's upstream 
visual design, and is currently a flat #222226 in HTML notation; older 
GNOME releases used a "noise" texture with some random variation around 
a similar average colour. Debian branding is shown below the list of 
users, and should be clearly visible. In bookworm we used 
/usr/share/images/vendor-logos/logo-text-version-64.png which consists 
of a mid grey swirl, a mid grey Debian logotype and a light grey "12", 
and still looks good on trixie GNOME's background: 
https://people.debian.org/~smcv/temp/2025/trixie-branding/good.png

Compare with the light coloured logos used in distros that have GNOME 
as their default desktop environment and therefore gdm as their default 
login manager, such as:

* Fedora: https://discussion.fedoraproject.org/t/default-login-screen/94510
* Ubuntu: https://documentation.ubuntu.com/authd/en/latest/howto/login-gdm/
   (ignore the stuff about remote login providers, it's the screenshots
   that are of interest)

Actual result
=============

gdm3 shows a dark grey background, as above. We still use 
/usr/share/images/vendor-logos/logo-text-version-64.png for the 
branding, but now that image is a slightly translucent #1a1a1a, a dark 
grey that is very similar to the background and therefore has very low 
contrast: 
https://people.debian.org/~smcv/temp/2025/trixie-branding/bad.png

Do I assume correctly that it was intentionally changed to a dark colour 
so that it would look good in some other context where the Debian 13 
branding appears on a light background? The changelog just says "Remove 
version from logo" and the commit message just says "Prepare for 
upload", so I don't know the reasoning behind this.

logo-text-version-SIZE.png are temporarily missing the version number, 
for #1086954, but that's expected: I assume it'll be reinstated as part 
of the final Debian 13 branding during hard freeze.

Workaround
==========

Possible solutions can be prototyped by editing 
/etc/gdm3/greeter.dconf-defaults to set an arbitrary logo image, then 
restarting gdm3 (`sudo systemctl restart gdm.service`) or rebooting.

Possible solutions
==================

We could change gdm3 so it defaults to a different logo image, one that 
is a light colour and therefore visible on a dark background. 
logo-64.png (just the swirl) is suitable, for now 
(https://people.debian.org/~smcv/temp/2025/trixie-branding/light-swirl-only.png) 
but I think we probably do 
want the swirl + logotype + major version number?

And I'm not sure whether desktop-base aims to guarantee that 
logo-SIZE.png is light coloured but logo-text-version-SIZE.png is dark, 
which seems an odd distinction to make?

Another possible route is that we could change desktop-base so 
logo-text-version-SIZE.png is a light colour that looks good on a dark 
background, as it was in Debian 12. That would not be suitable for 
non-gdm contexts where it might be displayed on a light background, 
though.

Or desktop-base could provide vendor logos in two flavours - one for 
dark backgrounds and one for light backgrounds - so that GNOME can 
choose the dark-background one and be confident that it will look good?

Or desktop-base could apply an outline to the logos (a dark logo with a 
light outline or vice versa) so that they will contrast on any background?

I tried adjusting the picture-uri, picture-options and primary-color 
options shown in the default /etc/gdm3/greeter.dconf-defaults, but they 
don't have any effect - I think the dark grey is hard-coded as part of 
GNOME Shell's visual design, rather than being intended to be 
configurable (and we should remove the non-functional options from 
/etc/gdm3/greeter.dconf-defaults to avoid confusion). So if we wanted to 
use a background image, we would have to diverge from upstream by 
patching gnome-shell, which I'm not keen to do.

     smcv