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Self-response: Stylus is useful here[1]. There is a list of already more readable styles than the default that the community has prepared.

[1]: https://github.com/openstyles/stylus

I have used Stylus for CSS tweaks: https://addons.mozilla.org/firefox/addon/styl-us/

You can hit the "Reader" button on Firefox (ctrl-alt-r), I think Chrome also has that? Or an extension that does the same thing would probably work.

Bit simpler than writing an override or fetching the HTML.

There are CLI tools to fetch and submit solutions. At least one of them allows you to download the puzzle description as markup: https://github.com/scarvalhojr/aoc-cli

On thing you can do is fetch the html and extract the main content to display it in your favorite way, it's been stable for years, and you can use the (stable) session_id cookie, it's how many people approach the challenge.

I'd just create a bookmarklet or run JS in devtools: `document.body.style.fontFamily = "sans-serif";` etc