Skin temperature and skin color are unrelated. Why not select for thicker layers of subdermal fat, and an oil secretion to protect the skin? We have fat, oily skinned people in the world, too.
They supplement with fish oils. If I remember correctly, cold-water fish are the kind needed.
Eating livers and kidneys of any animal will do it, too.
Because of this, their skin is not forced to lighten for production of vitamin D. Their skin color is closer to the world-average brown.
Then why don't we see a gradual darkening again as we head north? Brown in the south, brown in the north, white in the middle. That's the predicted pattern.
What's really interesting, is that there really is no need to hang onto this argument in the face of the contradicting evidence. It doesn't disprove evolution, or give creationists an upper hand to admit that we don't know why white people are where they are and brown people are where they are. It's really nothing more, I think, than having the desire to show the evolution can answer every question about living organisms right now. That's why social Darwinism is so big right now, and why every time somebody discovers something, the scramble is to place it in the evolutionary framework as quickly as possible.
Social Darwinism is big now?
Maybe you meant evolutionary psychology or the like, but "social darwinism" was a set of ideas that reigned many decades ago and are not at all like what you describe.
But then Social Darwinism was a corruption of Darwin's ideas too.
Eskimos, and the Samis/Lapps in Finland.