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

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

diff --git a/data/gnome-shell-theme.gresource.xml b/data/gnome-shell-theme.gresource.xml
index 76aeaa5..8eaf32e 100644
--- a/data/gnome-shell-theme.gresource.xml
+++ b/data/gnome-shell-theme.gresource.xml
@@ -22,6 +22,8 @@
     <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>page-indicator-active.svg</file>
     <file>page-indicator-inactive.svg</file>
diff --git a/data/theme/gnome-shell-sass/_common.scss b/data/theme/gnome-shell-sass/_common.scss
index 54d9f39..b9badd0 100644
--- a/data/theme/gnome-shell-sass/_common.scss
+++ b/data/theme/gnome-shell-sass/_common.scss
@@ -1742,7 +1742,16 @@ $legacy_icon_size: 24px;
 
 }
 
-  .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; }
@@ -1889,9 +1898,10 @@ $legacy_icon_size: 24px;
 }
 
 #lockDialogGroup {
-  background: #2e3436 url(resource:///org/gnome/shell/theme/noise-texture.png);
-  background-repeat: repeat;
-}
+  background: #2e3436 url(resource:///org/gnome/shell/theme/guix-background.png);
+  background-repeat: no-repeat;
+  background-size: cover;
+  background-position: center; }
 
 #screenShieldNotifications {
   StButton#vhandle, StButton#hhandle {
ass='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=89da127035737bdf922bc566970c5506c2e01b00'>root</a>/<a href='/guix/log/gnu?id=89da127035737bdf922bc566970c5506c2e01b00'>gnu</a>/<a href='/guix/log/gnu/packages?id=89da127035737bdf922bc566970c5506c2e01b00'>packages</a>/<a href='/guix/log/gnu/packages/markup.scm?id=89da127035737bdf922bc566970c5506c2e01b00'>markup.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/markup.scm?id=89da127035737bdf922bc566970c5506c2e01b00&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-11-20 10:02:48 +0200'>2019-11-20</span></td><td><a href='/guix/commit/gnu/packages/markup.scm?id=8840971767b9cfa78e204a607addcff2292a5026'>gnu: cmark: Update to 0.29.0.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/markup.scm (cmark): Update to 0.29.0.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2019-11-20 10:02:48 +0200'>2019-11-20</span></td><td><a href='/guix/commit/gnu/packages/markup.scm?id=2c24b96b1013f4dddd97df68b4af8aa68181e8e8'>gnu: cmark: Don't use unstable tarball.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/markup.scm (cmark)[source]: Download using git-fetch.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2019-11-20 10:02:47 +0200'>2019-11-20</span></td><td><a href='/guix/commit/gnu/packages/markup.scm?id=d71af69d1b02a304a8c5467bbd5a04e23e16fed0'>gnu: hoedown: Don't use unstable tarball.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/markup.scm (hoedown)[source]: Download using git-fetch.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2019-09-28 05:36:12 +0200'>2019-09-28</span></td><td><a href='/guix/commit/gnu/packages/markup.scm?id=62b3a9b4c1a0694feb8758be908018e390170f86'>gnu: smu: Don't use unstable tarball.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/markup.scm (smu)[source]: Use GIT-FETCH and
GIT-FILE-NAME.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2019-09-28 05:36:12 +0200'>2019-09-28</span></td><td><a href='/guix/commit/gnu/packages/markup.scm?id=67e4abc53115ad81579f1ea38e4aabfee1060b5a'>gnu: smu: Update to 1.5.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/markup.scm (smu): Update to 1.5.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>