Google introduced a new automatic dark theme with the release of Google Pixel 2, but it sort of depended on the wallpaper you choose. There was no toggle to switch between dark and light themes, but this manual selection may be coming with Android P build this fall. The option to manually select dark and light themes may be added with Android P, giving users the opportunity to choose the theme they prefer, and not rely on wallpapers.
This new feature was marked by a Google employee as fixed on the bug tracker page. He also replied, "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."
This means that with Android P, users will not have to rely on a wallpaper to switch to a dark theme. Currently, if you change the wallpaper to something dark, the phone automatically switched to a dark theme, and this includes a dark app drawer, black Quick Settings page, and dark folder icons . Similarly, if you choose a light wallpaper, the standard white UI elements would be displayed.
Apart from the dark theme toggle, Android P is said to introduce many new features like adaptive battery, adaptive brightness, new approach to navigation, UI changes, app actions, slices, android dashboard, and many others. The new features can be read in detail here. Furthermore, Google recently released Android P Beta 2 (aka Developer Preview 3) for compatible devices, which essentially smoothened rough edges, brought small tweaks to the app switcher, app drawer, and even the general layout. These beta releases, over the course of the coming months, will continue to shed light on what is coming with the commercial release of Android P in October.