I haven't solved it yet, but I agree that the Amazon Fire 7" is still fairly bright even with the light all the way down. One solution is to have your room brighter as it's the difference that gets to your eyes.
On my iPad I have Marvin which has a lot of choices for background color and font color allowing me to find a suitable contrast level. Simply having white text on black background can still be quite hard on the eyes.
Here are 4 android apps for dimming the screen:
https://www.maketecheasier.com/scree...-android-apps/
Haven't tried them yet