Right now, the profile has a single property "topcolor" where you can slightly customize HN. It seems that this could easily be extended with a few more similar properties, suitable values of which could produce a darkmode.
However, someone who is plugged into Reddit told me that they used to allow user-defined CSS for customizing subreddits and had to roll it back because of security issues. They built an entire customization UI instead. That scared me off the idea.
My fear with a customization UI—the HN equivalent of which would be profile settings, as you indicate—is that it would get way too complicated too quickly. How many additional settings would we need?
If someone can define a not-too-complicated set of profile settings which, when filled in to taste, would solve people's dark-mode concerns, I'm all for it. We could even make a separate editor page for it and link to that from the profile. The other constraint is: for each setting, the input format would need to be simple to sanitize (e.g. 'valid hex color'). Otherwise we just create the security issue all over again, only with a shitty nonstandard application-level CSS editor instead of standard CSS.
There could be cross-site scripting attacks whereby the user clicks on something evil in a malicious site which attacks that user's HN authenticated session by installing some malicious CSS in their profile. The UI could require a confirmation of the account password for updating any security-sensitive properties.
Users could also shoot themselves in the foot with bad CSS. The obvious fix for that is that the profile settings page itself doesn't inject the CSS, or there is a safe variant of its URL which doesn't inject CSS.
I posted a possible solution in another comment here[0]. You would have to write a separate dark theme, and there would be a single option to select it in the user panel. The data is literally just an integer pointing to an index in a whitelist of css filenames, and any errors would default to displaying the existing stylesheet.
P.S. I hurried to this thread because I noticed that the ‘collapse comment’ links became larger on my phone and I can finally hit them on the first try. Thanks to whoever did that, sincerely! Though personally I'd encourage you/them to make the link three times wider still, to take away even more effort. Dunno if it's much visual noise to others, but at least I don't think it encroaches on other elements.
Btw, another hopefully-not-controversial change that I would make is to have the post text black, not gray. It's a pretty important element, isn't it? So it should have high contrast and be readable.
For example, I’d hide the “hide” links on the main page. I’m sure some people love that feature, but I’ve only ever used it accidentally (fatfingering on mobile).
I’d add a bunch of additional hiding CSS. I’ve long wanted a “helps-me-respect-my-time” version of HN, where voting buttons, flag links, reply boxes, and reply links are all hidden.
What.cd used to let you put a url in your settings that was injected as the stylesheet while you were logged in. This approach would let people publish and share themes--though shared themes could be a (pretty visible) security concern.
I remember also running into issues trying to use non-https-served stylesheets since the site was https and browsers balked.