You need to add controls, as some people would like it set up differently than their OS. You need to store the setting. You need to consider people without accounts. You need to put a bit on thought into the color scheme, as this website is after all known for its color. In the end, many people will complain, regardless of how well you do.
I can virtually guarantee that the amount of people getting annoyed at being flashbanged by HN is larger than the amount of people that would complain about a dark mode.
For users without an account you just stick to prefers-color-scheme. For users with an account you add a setting 'disable dark mode'
Dark Reader has autodetection so those users won't be a problem either.
And if you really wanna keep to the identity of the site, the top bar doesn't even really need a color swap.
It really is less of a conundrum than you think.
> You need to add controls, as some people would like it set up differently than their OS.
The Browser also has controls. Good Browsers let you set your browser-wide choice differently from your OS-wide choice. Great Browsers let you pick per-site overrides directly, as a standard user setting in a consistent location in browser controls. I realize a lot of UX designers have come to much prefer the "add more controls" approach over the "teach a person to fish" / understand how your OS and browser controls work as the user of the site approach. I realize why a lot of UX designers will always prefer that approach, because teaching people is hard and it is easier to cut complaints off at the pass than answer complaints with "use your browser's settings".
But seriously, it should be fine to release a dark mode in 2025 that only responds to `prefers-color-scheme: dark` and leaves it to users to understand their OS and Browser tools. It irks me a lot more when sites like Wikipedia and Bing and Google ignore `prefers-color-scheme: dark` by default and makes you dig for some dumb website-specific control (that's in a different place on every website) just to set it to whatever they call "System default" that means "trust the Browser's prefers-color-scheme, I know what I'm doing". UX designers have taken something that should be natural and automatic and made it more complex and more confusing just because a small handful of users complain that they don't understand their OS and Browser Settings tools.