theme: re-render to fix modal headings

https://bugzilla.gnome.org/show_bug.cgi?id=745687

data/gnome-classic.css

@@ -325,7 +325,7 @@ StScrollBar {
 
 .prompt-dialog-headline {
   font-weight: bold;
-  color: black; }
+  color: #bebeb6; }
 
 .prompt-dialog-description:rtl {
   text-align: right; }