What Drow ought to look like
In this picture I have also given the Drow girl large, bat-like ears as I assumed that her hearing would need to be excellent in such dark surroundings. She may even be blind. I postulate this with the assumption that the Drow, after thousands of years in the Underdark, cut off from the Surface, would adapt to their environment.
Further comment: Thank you for some great discussions on this subject! Some of the comments contained points so important that they ought to be mentioned here.
1) Talking of evolution in this context in misleading, as the process takes a lot longer than the few thousand years I mentioned. This becomes even more true if one assumes that the Drow have a longer life-span and reach maturity later than Humans.
2) A self-contained world COMPLETELY without sunlight is not orthodox D&D, whichever version of the rules one looks at. I know this and I only discuss the make-up of life without sunlight because it is an amusing theoretical dilemma.
3) There are several different versions of the D&D world. In one early version, the Drow were in fact pale and in another, dark only refers to their moral alignment.
An idea for Drows and the Underdark: for some reason, some organism begins to "seismosynthesize", basically turns abiotic and biotic compounds into sugar and oxygen using micro-seisms. They mostly live in the underground since on the surface they are outcompeted by plants (those naughty green things) and fill a similar niche to them.