Christopher,
I'm not quite sure on what you are trying to accomplish but, you can use the developer tools in chrome or firebug in firefox to see the css tags.
Hope this helps you further.
The buttons that are in the mendix navbar are a different element to the drop down menu buttons and therefore don't seem to have the button icons. What you could do however is style the background to have a background image using the class: dijitArrowButtonInner. This would make all the buttons have the same background. In order to make certain buttons have different backgrounds you could use the an id css style reference and guess the auto generated id of the button. Usually they have this format dijitformDropDownButton_0. So if you know what the high level navigation are always going to be you can do it this way. So something like this:
.mendixNavbar .MxClient_navbar #dijit_form_DropDownButton_0 .dijitArrowButtonInner {}