Whoops, I forgot to enable the dark theme on the page; maybe the need to explicitly enable using the browser’s dark theme is why Safari doesn’t really bother. I’ve switched the dark theme to use Firefox’s colors, let me know if there’s anything that is still broken.
The addon version works on Android.. There’s an issue/“idea” for Chinese support as well. If I recall correctly, though, one of the problems is tokenization (the languages currently supported for example have spaces to separate words).