If the further north you live, the lighter you are, and the further south you live, the darker you are, why?

Ok, so I've heard many people talk about how northern whites are lighter than southern whites and blacks are dark because they're in Africa and the reason the white race evolved is because a black person went over to Europe and got really light. So, if all this is the case, how come I'm not getting any lighter? I live in Wisconsin and during the summer, I spend all my time in the very cold basement and rarely leave it and only come out to hang out with friend during the night hours because I can't stand the sun nor the heat. For the few times I am actually in it, I shade myself. So my question is, why am I not getting any lighter if the aforementioned ertion are true? I don't want to be lighter by the way; I just want to know why I am not getting lighter based upon the previous ertions. On a scale of 1 to 10 with 1 being light and 10 being dark, I have always been a 4, on the lighter side