It would not affect racism at all, until it became as commonplace as plastic surgery is today. That is to say, not until it's common enough that most people have someone close to them that has used this kind of gene modification. Even then I expect you would still have quite a bit, as most people I've met who were racist, would never bother to find out what you were before gene therapy. They would just see skin color in the first place, and not bother beyond that. Maybe you would get a large number of black people trying to look more white. But that would only make racism worse, as those who did not undergo gene therapy would catch even more.