Old MacBook has turned dim over time: what part should I replace?

I recently had my old white Macbook returned to me by my sister who has been using it for some time. I noticed that the screen has turned quite dim/dark over time. There's no "trauma", it never fell and was never damaged in any way; the screen just got naturally darker.

So, what should I replace? Do I need a new inverter? What's the most likely necessity to get the screen nice and bright again? Or, should I just buy a whole new screen from eBay and replace it entirely? (That sounds like it might not even work, because who knows how bright that new screen would be.)

Thanks for any help you may offer.