Index: wp-admin/css/global.dev.css
===================================================================
--- wp-admin/css/global.dev.css	(revision 17323)
+++ wp-admin/css/global.dev.css	(working copy)
@@ -393,6 +393,10 @@
 	text-decoration: none;
 }
 
+.widefat th a{
+	background: none;
+}
+
 .widefat thead th:first-of-type {
 	-moz-border-radius-topleft: 3px;
 	-khtml-border-top-left-radius: 3px;