Markdown files appear in light grey over dark grey

I’m on MacOS. I have a box folder with some markdown (.md) files. On both Firefox and Safari (the only two browsers I’ve tried) when I open them, the text appears in light grey over a dark grey background. It’s impossible to read, so I have to download the files and open them if I want to read them. Is that a known bug? (I’m using the light theme on my Mac, if that matters).