On macOS 10.13 High Sierra and earlier
The Dark Mode is only available on macOS 10.14 Mojave.
On macOS 10.14 Mojave and later
1. On Installation: You’ll get asked for the appearance as part of the installation of Mojave.
2. After Installation: If you want to change it afterwards go to System Preferences > General and change the Appearance.
2b. On macOS Catalina:
You get even a third option to change the appearance automatically based on your Nightshift preferences.
