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 {
ath'>path: <a href='/guix/log/?id=a1205efe1130d4084c6205a89108f1b1743ba3d5'>root</a>/<a href='/guix/log/tests?id=a1205efe1130d4084c6205a89108f1b1743ba3d5'>tests</a>/<a href='/guix/log/tests/guix-authenticate.sh?id=a1205efe1130d4084c6205a89108f1b1743ba3d5'>guix-authenticate.sh</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/tests/guix-authenticate.sh?id=a1205efe1130d4084c6205a89108f1b1743ba3d5&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2014-04-22 11:30:58 +0200'>2014-04-22</span></td><td><a href='/guix/commit/tests/guix-authenticate.sh?id=6f69588529f9898dc4f2defd21603cc4abbaca17'>authenticate: Allow signatures with binary data to be written to stdout.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2014-03-22 22:57:10 +0100'>2014-03-22</span></td><td><a href='/guix/commit/tests/guix-authenticate.sh?id=9b0a2233dbe8746015b97e97cef890d38fb44b0f'>authenticate: Support reading the hash or key from stdin.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
tring distance.</a><span class='msg-avail'>...</span></td><td>zimoun</td></tr>
<tr><td><span title='2021-02-01 17:32:34 +0100'>2021-02-01</span></td><td><a href='/guix/commit/tests/utils.scm?id=579506e272cf68649ec68ad8a976a17426ea630c'>utils: Add 'version-unique-prefix'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-01-13 22:24:19 +0100'>2021-01-13</span></td><td><a href='/guix/commit/tests/utils.scm?id=db0cecdf6b2f2b8f9c5a3cebe8fc60e79a692be0'>utils: Support zstd compression via Guile-zstd.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-01-13 22:24:19 +0100'>2021-01-13</span></td><td><a href='/guix/commit/tests/utils.scm?id=0d046587107a56467cf2027799ac79ce8c203ce0'>utils: Remove 'compressed-output-port'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-08-24 13:35:24 +0200'>2020-08-24</span></td><td><a href='/guix/commit/tests/utils.scm?id=4c0c65acfade63ce0549115d19db4b639c1e9992'>Use "guile-zlib" and "guile-lzlib" instead of (guix config).</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>