We've got another tidbit from our time at Google's Android office hours here at I/O, and you're probably not going to like it, Nexus owners. Speaking to Dan Sandler, we were told that night mode and the dark theme were both simply not up to the standard of performance and polish that Google wants to see of features in this release. As such, according to Dan, it is very unlikely either feature will ship when N is released in final form later this year.
We've received dozens of tips about the night mode and dark theme disappearing in Android N Developer Preview 3 (kind of, it still lives on in the quick settings tiles), and it sounds like the reason it's gone is indeed because Google isn't planning to ship it. Anything can happen, of course, and it wasn't ruled out to us as a matter of certainty - just that either feature showing up in N again was exceedingly unlikely.
Elaborating, Sandler and other Googlers stressed that features in the system UI tuner - where night mode and dark theme live - were subject to change or removal at any time, as in the disclaimer when you launch this part of the UI for the first time. Dark theme was a matter of consistency, at least to some extent, where getting it to look right across all parts of the OS and every app was difficult. As for night mode's color shifts (adjust tint/brightness), Sandler said it really just came down to the way it had been implemented not being great. Performance was poor because it used the accessibility features of the OS, which are notoriously slow, and as such the feature wasn't deemed up to snuff.
Now, will night mode and dark theme return someday? Google clearly had an interest in developing these features, so it doesn't seem farfetched to suggest they might be working on better, faster, more robust versions of these things. But it's very unlikely we'll see the fruits of those hypothetical labors in Android N.