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 {
/<a href='/guix/log/.dir-locals.el?id=7cb7be17af77caa084c1b5ebf20748dd04f208fa'>.dir-locals.el</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/.dir-locals.el?id=7cb7be17af77caa084c1b5ebf20748dd04f208fa&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-03-07 00:00:18 +0100'>2019-03-07</span></td><td><a href='/guix/commit/.dir-locals.el?id=a4678c6ba18d8dbd79d931f80426eebf61be7ebe'>database: Make 'register-items' transactional.</a><span class='msg-avail'>...</span></td><td>Caleb Ristvedt</td></tr>
<tr><td><span title='2019-01-11 12:14:38 +0100'>2019-01-11</span></td><td><a href='/guix/commit/.dir-locals.el?id=7804c45b9ce5a8edd06452d828249e588ae26263'>status: Add 'with-status-verbosity'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-10-19 18:28:57 +0200'>2018-10-19</span></td><td><a href='/guix/commit/.dir-locals.el?id=19c924af4f3726688ca155a905ebf1cb9acdfca2'>tests: Run 'guix pack' tests using the external store.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-09-27 23:21:53 +0200'>2018-09-27</span></td><td><a href='/guix/commit/.dir-locals.el?id=dc0f74e5fc26977a3ee6c4f2aa74a141f4359982'>Add (guix status) and use it for pretty colored output.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-06-01 15:35:32 +0200'>2018-06-01</span></td><td><a href='/guix/commit/.dir-locals.el?id=7f9d184d9b688d13ce76eefabaddcfa76bdde2b5'>Add (gnu store database).</a><span class='msg-avail'>...</span></td><td>Caleb Ristvedt</td></tr>
<tr><td><span title='2018-06-01 15:20:54 +0200'>2018-06-01</span></td><td><a href='/guix/commit/.dir-locals.el?id=838e17d8050236a6d3ffde991fb0035412eb3046'>gexp: Add 'with-extensions'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
h".</a><span class='msg-avail'>...</span></td><td>Hartmut Goebel</td></tr>