I don't like Rivain, like the reason why they are here on Thedas. They are here to be the tolkein blacks. They are criminals, and their race is always in poverty. Apparently black culture is a ghetto, or the times in the Colonial/Victorian era when they were part of pirate crews with no homeland since they would be caught and terrorized where ever they stayed.
Thedas is Europe in the Medieval era, the blacks in Medieval Europe where the invaders from north African, the Muslims, the Moors, and they were not what the Rivaini are, but what the Qunari are. The Rivaini represent the Medieval Spanish rather than the medieval blacks. Its hard to accept, maybe that is why they were changed. In DAO it is clear that Duncan and Isabella don't look black, but looked rather Spanish. And Sten was the only person who looked a black.
The media is wrong
The only Rivaini criminal I recall is Isabela.
Actually, what we do know is that the Rivaini show a great deal of respect for elves in comparison to the other nations of Thedas, they aren't fervent believers of the Chantry and they were in fact allowed to let their mages wander free and do as they please, until the recent troubles arose. They're possibly the most socially enlightened and respectful nation in all of Thedas.
There was no longer a need for it but there is also no clear reason why humans would lose it. There isn't really a significant benefit to not having it in cold climates and not every cold climate is populated with white people. Darker skin and hair genes are also more dominant and traits don't just leave when you no longer need them.
The sun is just as good to us as it is bad. Although it has the potential to harm and damage us, exposure to the sun does stimulate the creation of important nutrients your body requires for a healthy life. As an example, Vitamin D is required for strong and healthy bones and the vast majority of Vitamin D in our bodies is created by exposure to sunlight. A lack of Vitamin D can lead to rickets and osteomalacia. In short, soft, weak and deformed bones.
Now for the interesting part.
The paler you are, the better your skin is at producing Vitamin D. The sun's strength also has an impact on Vitamin D production. In cold and cloudy regions, darker skin isn't adapted to produce Vitamin D while paler skin is. In hot and sunny regions, paler skin isn't adapted to enduring the onslaught of the sun's light while darker skin is. As such, when humanity migrated away from the equator, those with naturally darker skin would have had a harder time growing up than those with paler skin, with bone deformities caused by the lack of Vitamin D. This would have made it much more difficult for them to survive, let alone breed while those with naturally paler skin had a much better time in the colder regions of the Earth, generating Vitamin D much more easily and suffering no deformation.
As such, my personal theory is that darker skin was, well... bred out the further you go from the equator, due to paler skin being more efficient at generating Vitamin D. Even in the modern day, it's advised that darker-skinned children are given Vitamin D supplements when growing up in order to stave off any potential weakness in the bones.
Sorry for the tangent!