11 February 2019
Google has been implementing dark mode in its applications like YouTube, Phone, Contact, and Messages etc. Tough the users have been requesting for a system-wide dark mode since some time and it looks like that with Android Q, we may get one.
Currently, most apps use light colors, mostly white, in accordance w the Material Design by Google. Lighter colors drain a huge amount of battery and can be strenuous for the eyes as well. Google seems to have realized the OLED panels can save a lot of battery by using a dark mode which otherwise was available in the Android SystemUI, Framework, and most apps before Android 5 Lollipop.
A Googler has confirmed the feature in a post on Chromium bug tracker:
Dark mode is an approved Q feature [...] The Q team wants to ensure that all preloaded apps support dark mode natively. In order to ship dark mode successfully, we need all UI elements to be ideally themed dark by May 2019.
Dark Mode expects a specific wallpaper file to be present in the filesystem. This file is generated for most ways that the wallpaper can be set except for when the default_wallpaper resource is used.
The descriptions suggest that the dark mode would be available under Settings > Display > Dark Mode. Though a lot of Google documents describe the feature, however, these are now inaccessible. Some of the threads are dated old as of October 2018, though it does suggest how serious Google is about this feature.