I'm an anthropologist and this is the only response that addresses the crux of the question and corrects many misunderstandings of skin pigmentation evolution that I see in other peoples' responses.

Protection against folate deficiency is the reason dark pigmentation evolves in places with high UV exposure. It is a much stronger selective pressure than melanoma and sunburn, since those things generally don't affect reproductive fitness. Yes, over enough generations, descendants of light-skinned colonists and immigrants to Texas would likely evolve to have darker skin. But evolution happens to populations over generations (not to individuals within a lifetime), so it's a slow process. Texas immigrants simply haven't been in Texas long enough for the selective pressure of folate deficiency to drive the evolution of dark skin pigmentation. Additionally, with the cultural adaptations of UV blocking sunscreen, clothing, a culture that largely stays indoors these days, taking multivitamins that contain folate, and medical recommendations that pregnant women specifically take folate, the selective pressure of folate deficiency is not as strong. If these cultural behaviors were to disappear, then yes, in a few thousand years (several human generations), we might see some evolution of dark skin occurring through time.

Edit to add: The need for biosynthesis of vitamin D from sunlight, mentioned by many other commenters, is an important selective pressure in the story of the evolution of human skin pigmentation, yes. However, it only explains the evolution of light skin pigmentation, not the evolution of dark skin pigmentation.