Voting

You could support this by making vote buttons submit a form if JS isn't enabled. (That's what mlmym does.)

Can't manually switch between dark and light mode

Hmm... There are some pretty nifty things you can do with a hidden checkbox, label, and some clever CSS (e.g. html:has(#element:checked) + CSS variables -- though FYI :has is baseline 2023.)

Making it persistent would require some more effort -- e.g. form + cookies + server side style sheet selection, most likely. mlmym lets users change their theme w/o JS by submiting a form on the setting page. I'd have to think a bit if there's a good way to make it persistent across multiple requests for logged out users with a CDN caching things in between though...

only automatically based on browser settings

Doesn't actually work for me in a FF138-based browser w/ JS blocked via NoScript -- I always get light mode despite having a dark mode preference set. (Where do you have your prefers-color-scheme media query?)

Also, FYI I had to manually override font restriction -- otherwise all your buttons end up as tofu characters. (I think NoScript is being kind of unreasonably strict there by blocking first party fonts.) That's a papercut kind of issue, but figured I'd point it out in case it might save you some debugging time if you get confused NoScript users in the future.