You can somewhat change the appearance in System Preferences, under Personal, Appearances. You may have been seeing reverse-color mode, which you can enact by pressing apple-option-control-8.
Nope. There are third-party tools, like ShapeShifter, that allow you to use themes, though.
It's possible you just saw the display inverted. You can do that in OS X by going to System Preferences, clicking Universal Access, and changing the display option from "Black on White" to "White on Black". I find it useful on occasion.