I used to loathe websites with light text on black backgrounds. It's terribly painful to look at. Dark text on light backgrounds is much better. But then, I talked to other people and learned that not all eyes work the way my eyes work. That things that are painful for me to look at, aren't painful for everyone to look at. And things that aren't painful for me to look, are actually painful for some other people.

So to get around this, some apps and social medias and websites and other things tend to have a Day Mode and a Night Mode. Dark text on light backgrounds and light text on dark backgrounds. Offering choice since one size cannot fit all.

Do RPG rulebooks need to come in pairs? A Day Mode PDF and a Night Mode PDF? Is that something that could make texts more readable?

Sort of, but not the way you mention.

eReader devices and most software readers have that capability built-in. It's a good idea to test your rulebooks to make sure they work in both situations. Images including dark text on transparent backgrounds or light text on transparent backgrounds are the biggest culprits, as the background will change dynamically, but the image may not. I tend to put a stroke around text that needs to be in images like that.

For accessibility, PDFs are kind of terrible. ePubs or other non-fixed layout formats are better -- for the aforementioned day/night swap, but also being able to zoom the text without having the scroll all around the document with a virtual magnifying glass. If you also want to get real into it, try feeding your game into a screen reader. It's an experience.

That said, all this comes along with trade-offs in visual layout control (as a lot is now in the hands of the user). It's up to you as a designer if that cost is worth it. For me, it usually is.

Great topic. Accessibility in texts in general is  a huge problem many TTRPGs don't succeed at. Here is a good font resource. The British Dyslexia Association has a style guide which can also help with visual stress from reading no matter whether the reader is dyslexic or not.