Is there any kind of static site generator that lets me write documents in Markdown, store them in a directory, and automatically loads them into the static site and translates them to CSS?

I say this out of a genuine desire to be helpful, I promise: you wouldn't be translating your Markdown "into CSS", you'd be translating it into HTML, and then optionally applying CSS

To your original point, if you want something really simple this might work (haven't tried it, just found it on Google): https://github.com/gomarkdown/mdtohtml

And then for CSS, you could drop in something like this: https://github.com/andybrewer/mvp