Dark mode is now everywhere (this blog has it). It’s both accessibility-driven and aesthetically pleasing. But it often isn’t the highest benefit feature (and rarely, if ever, a feature that leads to product-market-fit).
It’s not a technically difficult thing to build — across my entire blog, supporting dark mode takes up about 20 lines of code. For more complicated UIs, or retrofitting legacy applications, it’s a bit more involved (not to mention the technical debt of supporting it for future changes).
In their quest for product-market fit, most startups should prioritize features that bring them closer to that goal. Dark mode might marginally increase conversion and retention, but it won’t be a stepwise change. Good product managers must allocate resources effectively (even when personally partial to features).
After you nail the core features, you can build things that further delight users (marginal benefit > 0). Sometimes the details can be differentiating. Perfectly crafted products make a difference (but never forget the core value proposition).