EDIT – There is night mode, it now has its own icon in quick settings. Drop down the quick settings from the top, hit ‘edit’ then drag and drop the ‘Night Mode’ icon to your selection of tiles. It doesn’t offer a ‘dark mode’ however.
Last night, as well as launching the first public beta for Android N, Google pushed the third developer preview out and it includes some notable differences from the previous two.
Android N Dev Preview 1 walkthrough – https://www.youtube.com/watch?v=5NoB7jScnNY
Android N Dev Preview 2 walkthrough – https://www.youtube.com/watch?v=nxssSsg0GYQ