Use the guest.css for the maintenance page as well

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

core/templates/singleuser.user.php

@@ -1,10 +0,0 @@
-<ul>
-	<li class='update'>
-		<?php p($l->t('This Nextcloud instance is currently in single user mode.')) ?><br><br>
-		<?php p($l->t('This means only administrators can use the instance.')) ?><br><br>
-		<?php p($l->t('Contact your system administrator if this message persists or appeared unexpectedly.')) ?>
-		<br><br>
-		<?php p($l->t('Thank you for your patience.')); ?><br><br>
-		<a class="button" <?php print_unescaped(OC_User::getLogoutAttribute()); ?>><?php p($l->t('Log out')); ?></a>
-	</li>
-</ul>

lib/base.php

@@ -280,6 +280,7 @@ class OC {
 			// render error page
 			$template = new OC_Template('', 'update.user', 'guest');
 			OC_Util::addScript('maintenance-check');
+			OC_Util::addStyle('core', 'guest');
 			$template->printPage();
 			die();
 		}