In my opinion, the macOS Mojaves dark mode is one of the best features that Apple introduced for me. I work after hours a lot and the dark mode is awesome for that!
But I think Apple forgot to implement a automation or scheduler for that!
There is a freeware tool called NightOwl that can do the job. I just tested it and it works fine.
But in my workflow I need to do a bit more then just toggle the dark mode of macOS Mojave. Some Apps have the same (and many of then are still not so deeply integrated into macOS Mojave that they use the toggle).
So I came up with an poor mens approach that uses the macOS internal functionality (StartCalendarInterval with LaunchAgents)! to call a script that does all the thinks I want.