Google has been on again/off again with the concept of a dark mode in Android, but it seems they’ve found a compromise with a response to a request for Dark Mode in Android P being offered on the Google Issue Tracker.
The current implementation in the Pixel Launcher is to mimic the colour of the background wallpaper for the colour of options like your Quick Settings, App Drawer and folders. This change would allow you to simply apply a dark theme to the system no matter what colour your wallpaper is.
The issue, which can be found here, was logged in March this year, has been marked as ‘Fixed’ by a Googler who followed up with a comment advising the update would be available in a future Android Build. The full comment, which includes instructions on where the updated switch to a dark theme will be, reads:
As Android Police point out, this isn’t a fix-all for those wanting a full dark theme, but it is at least a start. There’s no definite date for when the change will be implemented, it could be as soon as Android P Dev Preview 4, or may come when the full Android P release goes live.