Look for a theme that offers some better colours with respect to the backgrounds. Yes, they do exist. Personally, I've been using a theme with a black background - a very dark grey, actually - and white text since back before the Nougat (Android 7) update. My background when accessing settings is black and the text is white. While there still might be a contrast issue in bright light it is better than the default colour setup, IMHO. Furthermore, in the phone Settings access the Accessibility options and then Vision. You can adjust font size under Screen zoom and font as well as changing the actual font used to be more legible. There's also the option to toggle on high contrast fonts which will border the font in a black border. Personally, on my setup I find that makes things harder to read but on a white background it might help.
A combination of all of the above things should allow you a better viewing/reading experience.