.rf-ddm-sel and .rf-ddm-unsel can be referenced in the css, but are being overwritten by an extra dropdownmenu.ecss.xhtml style!
I do suggest the documentation to be updated though, as there's no referrence to these properties or whatsoever and all one can google about them is the source in svn... or is there a reason for this? may be they're not meant to be used?
So the question would rather become, where does this style come from (may be from the skin?) and how to disable it? Please note that I am using plain skin so I would expected no styles to be applied. Also I have tried:
Hope someone can provide a hint.
Finally the solution was to write my own dropdownmenu.ecss class and put it in my WebContent/resources/css folder.
Afterwards, I found other posts where it is said that writing the <h:outputStylesheet> inside the <h:head> tag would change the order of precedence, though it didn't work for me.
Hope this helps somebody out there in cyberspace