The feature will be added in a "future Android build."
Google's Pixel Launcher is one of the best Android launchers around, and with the launch of the Pixel 2 last year, it introduced a feature that enabled a dark theme when applying certain wallpapers. Now, Google's going to give you manual control for toggling it.
We have added support for a Dark theme to be applied to quick settings and launcher under Settings -> Display -> Device Theme. It will be available in a future Android build.
It's unclear what the "future Android build" will be, and while we could see this added as soon as Android P Developer Preview 4, we should get it by P's public release at the latest.
I love the current implementation of the Pixel Launcher's dark theme being tied to your wallpaper, but even so, it's going to be really nice to have the freedom of choosing when it is and isn't applied. Also, with Google putting this under a new "Device Theme" section, might we see even more themes added down the road? Speculate away in the comments below.