hi,
importing exisitng full css3 skins would be hard i know (but simple ones maybe, with a tool saying what is compatible and what is not, to make port easy),
but at least it is usefull to have a repository of compatible skins for haxeui, easy to browser and import from.
adobe flex is very close to haxeui, im very suprised you dont know that much flex, as it looked as if it was a huge influence 
the flex skins as only importable zip directory of files, but mostly assets and css skin files.
the good thing for haxeui is that flex also use only a subset of css3 for skining, so maybe it is almost the same?
i dont remember exactly where to find them, but a quick search got me here: http://scalenine.com/gallery/
and even if some links are dead, there is still some living files like this one http://www.scalenine.com/themes/vistaremix/srcview/VistaRemix.zip
flex has to redraw anything from scratch and had no native skin possibility so skins were essential. the default one was pretty enough to have clean app out of the box. the native skin for haxeui should be enough for that too.
but being able to just plug another skin and get same app with completely different visual was very interesting and usefull in flex
thanks