Index: wp-content/themes/twentytwelve/css/blocks.css
===================================================================
--- wp-content/themes/twentytwelve/css/blocks.css	(revision 59114)
+++ wp-content/themes/twentytwelve/css/blocks.css	(working copy)
@@ -26,7 +26,7 @@
 	font-size: 0.857142857rem;
 	line-height: 2;
 	font-style: italic;
-	text-align: left;
+	text-align: center;
 }
 
 [class^="wp-block-"].alignleft,