Personally the white on white theme of The usaco website (not the guide but the official website) makes my retinas scream in agony, and switching tabs between it and a normal website feels like opening the curtains first thing in the morning.

So I don't know if this already exists (I certainly didn't find it anywhere), but I created a script for a usaco.org dark theme. It's janky, it's mostly ai generated, but as far as I know there aren't any better options.

The setup is extremely easy and well explained in the website, here's a link for anyone who's interested:

https://greasyfork.org/en/scripts/526910-usaco-org-dark-mode-by-friday-night

And here are some screenshots: