would it be possible to put a ds lite screen on my ds fat because i can hardly see the screen outside

No. The ndsl has a different ribbon cable on the top screen to the ds phats. Maybe try an led strip on the border of the screen or maybe the gbas backlight??

thanks for the help. But also, can i use the internal power supply to power to backlight if i was to install it?

Yes. I remember reading for doing it on a gbc

Thanks for all the help. This is for the ds tablet I'm posting since I only have 1 ds fat.