Index: wp-content/themes/twentythirteen/style.css
===================================================================
--- wp-content/themes/twentythirteen/style.css	(revision 23482)
+++ wp-content/themes/twentythirteen/style.css	(working copy)
@@ -1245,6 +1245,9 @@
 	padding: 0;
 }
 
+.mu_register {
+	max-width: 1040px;
+}
 
 /**
  * 5.4 Galleries