I know "could be made in 5 mins" is an exaggeration, but there's a very real long-term cost to adding thematic options like dark mode that HN just might not want to take on.
Any time you split your UI (for example, light mode and dark mode), it effectively doubles/multiplies all future UI testing you do from then on out (testing all new views/features on both light and dark mode). It can also add additional strain on bug reports / support, since you no longer know what UI configuration a particular user might've been using when they complained about something.
Slightly less of an issue on a site like HN where the UI is somewhat simple and static and changes happen at alligator speed, but still a long-term commitment to make (as opposed to 5 min of dev time).
(In case it isn't obvious, I'm a little salty about how much time I still spend testing/fixing my own site changes in dark mode over a year after adding it.)