One of the features that Google revealed that would be part of Android 6.0 Marshmallow was a dark theme. Basically this made the entire UI of the operating system darker, making it less painful on the eyes at night, or maybe you just prefer a darker look. There was even an option to make it automatic.
Basically it is a nifty feature but for some reason, Google has decided that they won’t be shipping it with the final version of Android 6.0 Marshmallow. This was confirmed by Google in a post on the Google Issue Tracker in which they wrote, “Our development team has looked into this feature request. This will not be included in the M release. However, this may be considered for future releases.”
It does seem odd that such a simple feature would be omitted from the final release. Unfortunately we do not know why Google has made this decision as we can only imagine it doesn’t seem like an overly complex feature. It would seem that many users are frustrated by this decision.
Some have pointed out that a dark theme would go a long way towards saving battery, especially on devices that use AMOLED displays. In any case maybe Google will release it in the future, but for now there are Android launchers and themes that can serve up as worthy, if not better, alternatives.