My first theory was that it was to "correct" both sides so she's seeing the world in a similar way with both eyes. For those of you who remember red/blue 3-d glasses, it was a slightly weird thing to wear. It made the world look both red and blue at the same time, and could be headache-inducing if you wore them all day. When Serini got troll-blood, it changed one of her eyes to troll-like, most probably with low light vision, but who knows in what other ways trolls see the world differently to the rest of us? So I assumed that she'd made a set of glasses that adjusted her eyesight so that the colour balance for her eyes basically matched to stop her feeling that red-blue effect.