Every new Android P feature we have found so far [Continuously updated - now with DP3]
Android P DP4 revises gesture navigation button design
Manual toggle for dark and light launcher themes debuts in Android P DP4
Android P DP4 gesture app switcher now has full-width slider and larger previews
Android P DP4 trims down the Direct Share targets to 4 instead of 8
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Google introduced the dark launcher theme on the Pixel 2 and 2 XL, eventually expanding it to the first gen Pixels with the Android 8.1 update. However, there was no way to change the theme manually—it was always based on the wallpaper. Google recently promised a toggle, and now it's here in the latest Android P preview.
The default setting in DP4 is still to change the launcher theme when you apply a dark or light wallpaper. If you don't like that, go to Settings > Display > Advanced > Device theme. You can pick between light, dark, and automatic. The device theme setting changes your quick settings, folders, and app drawer.
This theme setting is not the elusive dark system theme (AKA night mode). That's still listed in the dev options as of this release, but we have no idea if it will remain available in the final version. I have every reason to expect the launcher theme toggle will make it into the final Android 9.0 release, though.