Lol, I find it pretty funny that people want to instantly call out "plot doesn't make sense" or "it's a story reason that she gets her skin-color turned white/lighter," but immediately ignore the fact that she was BORN with the favor/blessing of the Dark Spirit King, and lived with it for a decade by now, but not just was literally blessed with it.
If this was in fact the case, then wouldn't this mean that the Light Spirit King is leagues stronger than the Dark Spirit King? A born blessing/lived with for a decade vs a blessing received on a whim, well, more accurately was properly chosen to receive, but it was a rather quick and casual decision. Doesn't that kind of sound a bit bullshit?
I think the author also kind of forgot this because whether it was their preference for lighter-skinned girls or just trying to represent the typical discrimination that POC regularly face, they made it extremely clear (for either regard) in the LN where one of the lines of dialogue after her change was something along the lines of "you're prettier now that you're white," or "I've thought of you as a girl now."
I never read the LN myself, so I can't confirm it, but this should be something easy to check for people more inclined to know or figure out, right? As much as I'd like to, it's not a major issue for me, but I'd rather she keep her dark skin.