Twitter started testing a dark UI mode several months ago, but it was a server side toggle on beta and alpha builds with no way to guarantee it would be on your device. The test must have gone well because today Twitter announced dark mode as an official feature of the Android app. This should be available on all installations of the Twitter app soon.
Because Twitter has ridiculous UI fragmentation across its various test builds, you'll find the night mode toggle in the navigation menu, which is accessed either by your profile photo or the standard slide-out icon. When you do find the toggle, just flip it on and night mode will be enabled. Unlike the dark modes in most apps, Twitter's isn't completely back. Instead, it's a deep midnight blue. I actually rather prefer this.
— Twitter (@Twitter) July 26, 2016
You can shut night mode off at any time if it's not to your liking, but it'll be easier on your eyes in the dark compared to the app's usual white interface. If the toggle isn't showing up for you yet, give it a little bit. This is Twitter, after all.
- Twitter support