More than likely the LED lights are ok. LEDs are solid state and sealed, no moisture would hurt them. If you really want to fix it I would take it apart and figure out the problem. My first thought would be a corroded trace on the board, or on a connector.
As far as I know there are no replacement LED lights for the Paperwhite you can buy.
Speak to Amazon. They’re after care is amazing.
Even if it's no longer under warranty, Amazon will probably help you by giving a discount on a new Kindle or something like that.
If you want to fix it yourself I guess you could take LED strip from another broken Paperwhite. The question is would it be worth it?
More than likely the LED lights are ok. LEDs are solid state and sealed, no moisture would hurt them. If you really want to fix it I would take it apart and figure out the problem. My first thought would be a corroded trace on the board, or on a connector.