In late November, an Android user consulted with Dark Mode / Night Mode on Google's problem tracking system and wrote:
Dear Sir / Madam,
Using the Dark Mode / Black screen on an OLED screen can greatly improve the Android phone's battery life. Hope to be able to see the Dark Mode or Night Theme under "Settings" in Android 9.0. At present, only the installation of third-party applications to activate this Dark mode.
This feature quickly spread between Google engineering teams, engineers wrote a few hours later
Our engineering team has added this feature and will be online in future Android version updates.
Update: This question is flagged as reopening on the issue tracking website.
The article wrote: Hello everyone, do you still remember me? The three pieces that I recently posted seem to have added Dark Mode. Unfortunately, however, the problem was prematurely closed because of poor communication with teams dealing with AOSP issues.
In a future version update we've added developer-oriented settings (via developer options) to trigger the -night UI mode qualifier, which makes it easy for developers to create and test applications that deploy Night mode. This qualifier is already on Froyo (SDK 8) on the platform and can be globally tuned via UiModeManager since Marshmallow (SDK 23). However, it is not possible to have a toggle switch in the setup options at this time.
If anything is comforting, then we can confirm that we will not include Hot Dog mode in future releases (that is, all UI elements are yellow and red.)