I don’t know if there would be any circuit diagrams available for it, but it is possible something is damaged on the backlight display. If you were going to attempt to repair this yourself I would suggest Louis Rossman’s videos on electronics repairs. He mostly does iphone and macbook repair, but has a really good free course on repairing electronics like that.
That being said without diagrams you would be fumbling around a bit unless there was an obvious burnt component or cracked solder joint on the circuit board. I suppose you could try to find one of those old school front light adapters that were popular on the Gameboy Advance as it was impossible to see anything on the screen at night? Unlikely it would fit as-is and would need to be modified, or if you wanted to get really creative and had access to a 3d printer you could DIY one.
Some of them were just like a book-light (which would probably decently as well), others had a built in magnifier too.