Last month some people started seeing a "Night Mode" toggle in the Twitter for Android app. This came after many have apparently asked for just such a feature that's supposed to make using the app in the dark a lot easier than with its standard white-filled theme.
The important thing to stress here is that up until today it seems like this feature was undergoing testing, and was thus only available to a few lucky users. Not anymore, though.
From this point on every single user of Twitter's Android app should see the new toggle, located in the navigation menu - which you get to either by sliding it out from the left or tapping on your profile photo. Do however note that its activation is likely a server-side affair, so it might actually take a few more days before you see it. Still, it's on its way.
Once you enable Night Mode you don't actually get a true black background for the UI, instead it's more like a dark blue hue. Anyway, it should also help AMOLED screens consume less power than in the normal mode.