[Bug 1715722] Re: Allow safe override of GDM3 theme

[Jean-Pierre van Riel]

I've noticed in 18.04.2 (upgraded from 16.04 LTS), with gnome-shell
3.28.3-0ubuntu0.18.04.4 and ubuntu-session 3.28.1-0ubuntu3, the *lock*
screen is still pinning to /usr/share/gnome-shell/theme/ubuntu.css.
Note, the login session screen is using the correct theme.

This is related: https://bugs.launchpad.net/ubuntu/+source/gnome-
shell/+bug/1732450.

As per https://bugs.launchpad.net/ubuntu/+source/gnome-
shell/+bug/1732450/comments/5, the nasty I did to work around it shows
the gdm3 / session lock screen theme is still pointed directly to
/usr/share/gnome-shell/theme/ubuntu.css instead of /usr/share/gnome-
shell/theme/gdm3.css.

I also checked the alternative symlinking, and indeed, `pop.css` was
supposed to be used, yet the lock screen used ubuntu.css.

```
$ update-alternatives --display gdm3.css
gdm3.css - auto mode
  link best version is /usr/share/gnome-shell/theme/pop.css
  link currently points to /usr/share/gnome-shell/theme/pop.css
  link gdm3.css is /usr/share/gnome-shell/theme/gdm3.css
/usr/share/gnome-shell/theme/pop.css - priority 20
/usr/share/gnome-shell/theme/ubuntu.css - priority 10
```

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https://bugs.launchpad.net/bugs/1715722

Title:
  Allow safe override of GDM3 theme

Status in GNOME Shell:
  Won't Fix
Status in gnome-shell package in Ubuntu:
  Fix Released

Bug description:
  Currently, in artful, GDM3's theme is hardcoded to be /usr/share
  /gnome-shell/theme/ubuntu.css

  In the Ubuntu default session, this is the same CSS file as the gnome-
  shell mode in /usr/share/gnome-shell/modes/ubuntu.json

  However, in other flavors, it makes sense to install another mode json
  and theme css file with a clear name. Take, for instance, pop-session,
  that installs /usr/share/gnome-shell/theme/pop.css and /usr/share
  /gnome-shell/modes/pop.json

  In order to supply a custom GDM3 theme, which is currently hardcoded,
  the ubuntu.css file must be replaced with this pop.css file. This
  breaks ubuntu-session, causing it to have a different shell theme than
  expected.

  What would be desirable is for the GDM3 CSS file to be hardcoded to
  gdm.css, which would be a symlink to the correct stylesheet for the
  flavor. This would allow multiple GNOME Shell sessions to be installed
  at the same time.

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