The first trick to tackling any new challenge is getting your technical references sorted. My advice to all developers is to watch the pertinent WWDC sessions, read the high-level documentation, and to immerse oneself in the Dark Mode aesthetic by perusing other applications that support it:
It goes without saying that every Mac developer should watch What’s New in Cocoa for macOS. This is always a great overview that touches on the major changes to follow up on in other sessions. For Dark Mode in particular, be sure to watch both the Introducing Dark Mode and Advanced Dark Mode sessions. These include a lot of excellent advice about both high-level design considerations and low-level practical uses of the NSAppearance API.
Apart from the reference documentation on NSAppearance, be sure to read the longer-form Supporting Dark Mode in Your Interface, and Providing Images for Different Appearances. These high-level guides will orient you to the types of work you will likely need to do in your app. Finally, the macOS 10.14 Release Notes include a number of details about Dark Mode, and particularly about special background blending modes and how they affect the user interface of an application.
Although dark interfaces are nothing new, Apple’s official take on it with macOS Mojave establishes specific aesthetic choices. You’ll want to become acquainted with the decisions Apple has made so you can make the right call in your own app. I recommend switching your macOS Mojave Mac to Dark Mode and running as many apps as possible in Dark Mode to get a sense for the prevailing aesthetics.
The vast majority of Apple’s own apps have been tastefully adapted, and a growing number of 3rd party titles are listed on the Mac App Store as Apps That Look Great in Dark Mode. I’m honored that one of my own apps, MarsEdit, is listed there.