notice_yesno() buttons now centered; merged from HEAD

theme/standard/styles_layout.css

@@ -182,6 +182,10 @@ img.grouppicture {
   padding:10px;
 }
 
+#notice .buttons {
+  text-align:center  
+}
+
 #notice .buttons .singlebutton {
   display:inline;
 }