Adjust GNOME-Shell so that it uses a Guix theme by default.

Patch by Ludovic Courtès <ludo@gnu.org>.

diff -ur a/data/gnome-shell-theme.gresource.xml b/data/gnome-shell-theme.gresource.xml
--- a/data/gnome-shell-theme.gresource.xml	2019-12-11 15:06:31.000000000 -0500
+++ b/data/gnome-shell-theme.gresource.xml	2019-12-21 00:31:55.895866241 -0500
@@ -19,6 +19,8 @@
     <file>no-events.svg</file>
     <file>no-notifications.svg</file>
     <file>noise-texture.png</file>
+    <file>guix-background.png</file>
+    <file>guix-logo.png</file>
     <file>pad-osd.css</file>
     <file alias="icons/pointer-double-click-symbolic.svg">pointer-double-click-symbolic.svg</file>
     <file alias="icons/pointer-drag-symbolic.svg">pointer-drag-symbolic.svg</file>
diff -ur a/data/theme/gnome-shell-sass/_common.scss b/data/theme/gnome-shell-sass/_common.scss
--- a/data/theme/gnome-shell-sass/_common.scss	2019-12-11 15:06:31.000000000 -0500
+++ b/data/theme/gnome-shell-sass/_common.scss	2019-12-21 00:33:49.495832135 -0500
@@ -1994,7 +1994,16 @@
   }
 }
 
-  .login-dialog-logo-bin { padding: 24px 0px; }
+  .login-dialog-logo-bin {
+      /* Make it a bit narrower than .login-dialog-user-list.  */
+      width: 12em;
+      height: 8em;
+      background: url(resource:///org/gnome/shell/theme/guix-logo.png);
+      background-color: transparent;
+      background-repeat: no-repeat;
+      background-size: contain;
+      background-position: center; }
+
   .login-dialog-banner { color: darken($osd_fg_color,10%); }
   .login-dialog-button-box { spacing: 5px; }
   .login-dialog-message-warning { color: $warning_color; }
@@ -2143,9 +2152,10 @@
 }
 
 #lockDialogGroup {
-  background: lighten(#2e3436, 8%) url(resource:///org/gnome/shell/theme/noise-texture.png);
-  background-repeat: repeat;
-}
+  background: lighten(#2e3436, 8%) url(resource:///org/gnome/shell/theme/guix-background.png);
+  background-repeat: no-repeat;
+  background-size: cover;
+  background-position: center; }
 
 #screenShieldNotifications {
   StButton#vhandle, StButton#hhandle {
f77e5d5fa0c4669efa1012fc526868229821912'>root</a>/<a href='/guix/log/gnu?id=7f77e5d5fa0c4669efa1012fc526868229821912'>gnu</a>/<a href='/guix/log/gnu/packages?id=7f77e5d5fa0c4669efa1012fc526868229821912'>packages</a>/<a href='/guix/log/gnu/packages/sync.scm?id=7f77e5d5fa0c4669efa1012fc526868229821912'>sync.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/packages/sync.scm?id=7f77e5d5fa0c4669efa1012fc526868229821912&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2018-01-19 17:28:31 +0100'>2018-01-19</span></td><td><a href='/guix/commit/gnu/packages/sync.scm?id=162a13740041f907f0ce5c2aa05b52b162b4e81a'>gnu: Consistently Write ‘file system(s)’.</a><span class='msg-avail'>...<span class='msg-tooltip'>It is the GNU way.

* doc/guix.texi (Build Systems, DNS Services): Write ‘file system(s)’.
* gnu/build/vm.scm (create-ext-file-system, create-fat-file-system):
Likewise.
* gnu/packages/backup.scm (dirvish, rsnapshot)[description]: Likewise.
* gnu/packages/check.scm (python-testpath)[description]: Likewise.
* gnu/packages/disk.scm (pydf)[description]: Likewise.
* gnu/packages/file-systems.scm (disorderfs)[synopsis, description]: Likewise.
(glusterfs)[description]: Likewise.
* gnu/packages/haskell.scm (ghc-directory, ghc-system-fileio-bootstrap)
(ghc-system-fileio)[synopsis]: Likewise.
(ghc-fsnotify)[description]: Likewise.
* gnu/packages/linux.scm (proot)[description]: Likewise.
(jmtpfs)[synopsis, description]: Likewise.
* gnu/packages/mate.scm (caja, caja-extensions)[description]: Likewise.
* gnu/packages/storage.scm (ceph)[description]: Likewise.
* gnu/packages/sync.scm (lsyncd)[description]: Likewise.
* gnu/packages/syncthing.scm (syncthing)[synopsis]: Likewise.
(go-github-com-zillode-notify)[description]: Likewise.
* gnu/services/nfs.scm (pipefs-service-type): Likewise.
* guix/scripts/system.scm (perform-action): Likewise.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2018-01-10 15:59:47 +0100'>2018-01-10</span></td><td><a href='/guix/commit/gnu/packages/sync.scm?id=9358ae8adf4fc3dbbe4164d0bc16d5ce11b41393'>gnu: lsyncd: Update to 2.2.2.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sync.scm (lsyncd): Update to 2.2.2.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2017-11-18 21:50:11 +0200'>2017-11-18</span></td><td><a href='/guix/commit/gnu/packages/sync.scm?id=5af27a82c3f95eaa560e2771b629a93693c7f442'>gnu: owncloud-client: Update to 2.3.4.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sync.scm (owncloud-client): Update to 2.3.4.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2017-11-18 21:50:08 +0200'>2017-11-18</span></td><td><a href='/guix/commit/gnu/packages/sync.scm?id=6563d58c7e94bee8b9beeefa929ab2e982d86fa0'>gnu: owncloud-client: Don't check for updates.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sync.scm (owncloud-client)[source]: Add patch.
* gnu/packages/patches/owncloud-disable-updatecheck.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register it.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2017-09-17 10:14:14 +0300'>2017-09-17</span></td><td><a href='/guix/commit/gnu/packages/sync.scm?id=5fb17ed92e910030b86cc6f1eec50a2c3b725b08'>gnu: owncloud-client: Update to 2.3.3.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sync.scm (owncloud-client): Update to 2.3.3.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2017-07-20 00:35:02 +0200'>2017-07-20</span></td><td><a href='/guix/commit/gnu/packages/sync.scm?id=dc05f6523eb424bfeb4ddd2bf157232b4a72ca77'>gnu: qsyncthingtray: Update to 0.5.8.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sync.scm (qsyncthingtray): Update to 0.5.8.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2017-04-26 22:04:01 +0300'>2017-04-26</span></td><td><a href='/guix/commit/gnu/packages/sync.scm?id=be58ffa4aa518686294726bdd83733eca4e6c03c'>gnu: owncloud-client: Update to 2.3.1.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sync.scm (owncloud-client): Update to 2.3.1.
[arguments]: Enable more tests.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2017-01-24 15:46:01 +0100'>2017-01-24</span></td><td><a href='/guix/commit/gnu/packages/sync.scm?id=7ee557a75a695dbbdfa5187bdf85b530e0f92fbf'>gnu: Add lsyncd.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sync.scm (lsyncd): New variable.
</span></span></td><td>Ricardo Wurmus</td></tr>