Fix login page, because only guest.css is loaded now

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

core/css/guest.css

@@ -624,3 +624,16 @@ footer,
 .push {
 	height: 70px;
 }
+
+.hidden {
+	display: none;
+}
+
+.hidden-visually {
+	position: absolute;
+	left:-10000px;
+	top: auto;
+	width: 1px;
+	height: 1px;
+	overflow: hidden;
+}