Chrome on Android, web version, the phone is set to dark mode, the browser inherits it. Pixel 5.
I don't know how I'll fix this (I've never really debugged mobile web specific stuff before), but I'll look into it this weekend.
There must be a way to fix it, since I don't see the issue when going to https://www.mathjax.org/#demo and entering something like this: $$\frac{1}{\sqrt{2}}$$
EDIT: I actually have no idea why this would be happening, unless there is some sort of metadata that I should be providing on the page to tell chrome how to enable dark mode on my content. I don't think I'm messing with the styling on the web version at all.
This is chrome 97.0.4692.98
I'd also think we move this from HN to some issue tracker?