> The only valid response to this question (a question asked on hacker news of all places)

Hacker News - a site where people make a sport of middlebrow dismissals of others' work and complaining about the features of other sites and software they very likely have never had a single PR accepted for.

The real answer is that PG probably just added to topbar feature to have a karma reward for users to work towards as a means of positive reinforcement (I haven't actually found a specific quote from him on that) and also that the current admins tend to be conservative about adding new features, so likely they don't believe a dark mode would improve the quality of the site enough to be worth the effort.