Change the default address book card heading colors to better match the theme.

Thanks to Stephen Walker for the patch.

mail/components/addrbook/skin/addressbook.css

@@ -217,8 +217,8 @@ treechildren::-moz-tree-image(GeneratedName, MailList) {
   padding: 1px;
   padding-left: 5px;
   margin: 0px 0px 1px 0px;
-  background-color: #CCCCFF;
-  color: black;
+  background-color: Highlight;
+  color: HighlightText;
   font-weight: bold;
   min-width: 30px;
 }