After announcing iOS 12 earlier this afternoon, Apple moved on to its macOS desktop operating system. Like with iOS 12, there aren’t any major jaw dropping additions to the mix with macOS Mojave (10.15). 
First and foremost is the introduction of a new Dark Mode, which brings a dark theme throughout the operating system. It is currently possible

Source: Hot Hardware – Apple Announces macOS Mojave, Devs Can Port iOS Apps To macOS In 2019