Well the very thought seems rather nebulous and open to be interpreted by the observer. So that can be anywhere from the one side as in there is the elements that makes it Fantasy, such as magic, mythological creatures/races, and the Medieval theme. Then there is the element of Dark, the gloom, doom, despair, and so forth. Now that is not to say that it can technically engulf another theme, while maintaining the Dark and the Fantasy. Although to put it simply, if one applied a little more thought into the term it can mean more than what it implies on the surface. (Either that or I just have a multifaceted and dirty imagination to see the duality of how Dark Fantasy can be interpreted.)