The way I see it, as our MC learns more about living and enjoying herself, the art gets softer. Think of the art style as a reflection of the MC. Sharp, dark, edgy at the start like our emotionless killer. And now its softer and less harsh because she has learned to feel happiness and wants to learn to enjoy this second life.
Or something. I dont really care about the art style change myself, as I didn't notice at first lol. Anyways, title drop on chapter 69, kek.