There are a few new important CSS rules coming in a few days. And while there will be a script run to apply these changes to all sites, it's possible that the script will be unable to apply the changes to some (like if you run your CSS from a custom location other than the normal location).

So the following new rules are coming in for DHTML Drop down lists.

.dhtmlddl = The div for the drop down itself.

.dhtml_ddl_listitem = The divs for each individual item in the rendered drop down list

.dhtml_ddl_listitem_highlight = the div for the item from the dropdown list currently being hovered over.

Please see the image here to further explain these: