Update guest.css

core/css/guest.css

@@ -156,7 +156,7 @@ form #datadirField legend {
 }
 
 #alternative-logins {
-	margin: 30px 15px 20px;
+	margin: auto;
 	display: block;
 	min-width: 260px;
 	max-width: 400px;