There is also Dark reader which also works quite well.

Although these extensions work well enough. They sometimes mess with stuff like rating graphs on profile at times. Sometimes Dark Reader also glitches for a second where the site flashes you with bright white, or some parts of the site are dark but others aren't (usually fixed by a refresh, but still). It would actually be nice if we could get an actual dark theme for codeforces instead of relying on an extension.

Not that unreasonable consider the mirrors for codeforces already have dark themes.