why are the chasnid swarthy?

I'm personally more inclined to go with the explanation of "the devs are imperfect beings who don't have a full understanding about how geography affects skin tone over large swaths of time and didn't consider this when assigning skin tones to the various ethnicities" than I am to give them the benefit of the doubt and say that they did consider it and came up for an alternate inuniverse mechanism that governs skin tone other than the biological one that does it in the real world.

To me it's easy enough to write it off as something that just didn't have much thought put into it rather than search for justifications for it because it isn't something that I particularly care about. Chasind are swarthy and live in a climate that doesn't particularly lend itself to that being a thing; who cares?

Come to think of it this is likely one of those situations where the writers & the art team designing the generic NPCs simply weren't talking to each other given that Morrigan is explicitly stated to "look chasind", i.e., implying that she has stereotypical chasind features, and she isn't swarthy at all. She's quite pale.