Index: wp-includes/css/editor.css
===================================================================
--- wp-includes/css/editor.css	(revision 24311)
+++ wp-includes/css/editor.css	(working copy)
@@ -256,6 +256,10 @@
 	line-height: normal;
 }
 
+.wp_themeSkin .mceListBoxMenu .mce_formatPreview a span {
+	background-color: transparent !important;
+}
+
 .wp_themeSkin .mceListBox .mceOpen,
 .wp_themeSkin .mceListBoxHover .mceOpen,
 .wp_themeSkin .mceListBoxSelected .mceOpen,