Twitter constantly updates their mobile apps and usually they are just bug fixes but some updates are major and usually warrant a press release like material design makeover for Twitter for Android.
Over a month ago, a Google+ user noticed that Twitter was testing a variety of changes to Twitter for Android which included the new material design makeover and a night mode theme. The night mode theme was particularly interesting since Google integrated it to Android so that developers could port it to their apps. However, that was actually available on an alpha release of Twitter for Android and now you can actually use it in a beta app just like what I have installed.
In this case, the version that I have is not an alpha but it is a beta version (ver 6.4.0 beta 467) that includes that sweet Night mode toggle. Accessing the toggle is simple: Just do a swiping gesture from right to left to reveal the hamburger menu and it has been added on top of the settings option. When you turn it on, it changes the app background from white to a shade of blue which is a weird choice which in theory should make it easy for your eyes when using it in low light conditions.
It is a cool addition to the Twitter for Android app and weird enough, the feature has not been spotted on the iOS version. We have seen other apps employ the dark theme/night mode switch like the WSJ app or Reddit where they actually show varying degrees of black instead of the bluish version employed by Twitter.