When Google first released the initial Developer Preview of Android N, there was some excitement surrounding the return of Dark Mode. A mode that, just as you'd expect, turns UI elements and themes into a dark grey sort of color, it was originally hoped for release in Android Marshmallow, but that of course didn't happen. Logically, users were hoping for the new feature to launch in Android N, which is now in its third Developer Preview, but that doesn't look as if it's going to happen either. According to comments from sessions at Google I/O, neither Dark Mode or Night Mode are where Google wants them to be in terms of overall performance, and for this reason might "miss" the final release of N.
This new piece of news is based around comments made by Dan Sandler from the Android team made during Office Hours at Google I/O 2016. The reason that Dark Mode and Night Mode are now unlikely to make the credits in the final version of Android N is that it's just not fast enough in Google's eyes. While this is true of the features right now, there's nothing to say that this isn't going to try in future releases of the Developer Preview and indeed for the final version of N, but right now it looks unlikely these two features will be in Android N. Given that Developer Preview 3, itself launched during I/O earlier this week, no longer contains the Dark Mode as we know it, and that this version is said to be a lot closer to final than the previous two, the evidence keeps on piling up.
As for Night Mode however, there is a little hope for this, during the "What's New in Android" session following the Keynote on Wednesday at I/O, Night Mode was explained to developers, how to use it and why they would want to. This isn't any sort of confirmation that Night Mode is to be there in Android N, but given that Night Mode doesn't have to be a system-wide change, only done on an app-by-app basis, this could make the cut for the final release.