Topic: Screen color fading on my DSi?

A while ago I payed for my DSi to be fixed (the battery had exploded and my younger brother had scratched up the bottom screen) and when I got it back I noticed that the color began fading from the screens. It's fine at first, but after ten minutes the color on the top screen starts to fade. By the time twenty minutes go by the bottom screen starts fading a bit too, and horizontal lines show up on both screens. If I play for more than half an hour it becomes unbearable, since all of the games I have use the top screen I can't play for more than maybe twenty minutes before frustration sets in.

Could it be because of the battery exploding, or maybe something they did while repairing it? I tried to go back to the store and tell them about it but they just told me that they didn't give refunds and that they went nowhere near the top screen. And how can I fix it? Would re-assembling it work or do I have to buy new screens?