The theory is that in order for less and less sunlight available as people moved north to make vitamin D, their skin lost melanin. In the early part of the last century (before vitamin D fortification of milk) an outbreak of rickets (D deficiency disease) occurred in Chicago among African American infants because they did not get enough sun.
As far as modern human evolution goes, the key is the isolation of a population. With all the genetic mixing nowadays, it would be hard for much genetic change over time to only affect certain populations.
I googled on this topic and found the following article: The Biology of Skin Color. What's interesting is that there are at least two real feedback mechanisms (ie. ones that affect survival) due to ultraviolet sunlight: vitamin B and D. Nowadays, women when they're anticipating pregnancy, are advised to take B in prenatal vitamins to help reduce neural tube defects. Vitamin D is the rickets.
The article says that the earliest humans in Africa (pre-modern humans) were lighter skinned but as they lost body hair became darker skinned to reduce exposure to strong ultraviolet sunlight. Why? Because UV light decreases the amount of B in a woman's body, leading to more neural tube defects in children. IOW, there was evolutionary pressure in favor of darker skin and inhibiting lighter skin. This pressure goes away in Europe.
The UV connection with Vitamin D has the evolutionary pressure favoring lighter skin in Europe because D is needed for strong bone growth. Dark skin blocks UV from the Sun, leading to weaker bones in lower-light climates like Europe.
This all sounds perfectly plausible. I don't have the biological credentials to judge whether its accurate or not. Still, are these two pressures strong enough to produce such significant changes in only 30K years???