While a dark mode option made by the website itself may be able to handle certain edge cases that the browser dark modes may miss, for most part part the browser dark mode features handle most cases quite admirably.
I feel that rather than going behind individual website developers, making the need known to the platform browser developers is a more scalable approach. Even if HN gets a dark mode, what about the websites it links to? Do you also plan to ping the the Devs there for a dark mode option?
From my experience multiple independent dark mode toggles for each website I visit are a pain to manage. I never use them. Dark mode is purely and globally handled by browser extensions.