On my phone I find the regular site difficult to use anyways, so I go with hn.premii.com, which has a dark theme.

For those who don't mind using a bookmarklet or using something like TamperMonkey, this bit of js does a pretty good job darkening most sites:

!function(d){d.head.appendChild(d.createElement('style')).innerText='html,img,video{-webkit-filter:invert(1)hue-rotate(180deg);filter:invert(.9)hue-rotate(180deg)}body{background:#000}'}(document);