This seems to be enabled server side meaning that you won’t need to update your app. According to the users who have received the feature, basically this seems to consist of a toggle that lets users turn it on and off. It is pretty straightforward and as you can see in the screenshots, it basically makes the app look darker.
Like we said, the feature is rolling out to some users. There are other users who are using the beta who have not received the feature themselves, so like we said, it appears to be server side enabled. It is possible that Twitter is merely testing out the feature on a wider scale, so if you don’t see it yet, don’t worry.
No word on when it is expected to make its way to all Twitter users (on Android and iOS), but for those of you who spend your nights in bed reading Twitter on your phone before you sleep, this feature could have your eyes thanking you.