Tweetbot is a popular iOS Twitter app, full of features and small tweaks that many consider an improvement over Twitter's own app.
One such feature is the ability to automatically change between the default view, which includes a predominantly white interface, and a night theme, complete with a predominantly dark interface.
The former is easier to see in normal, everyday use when you're in adequate lighting. The latter, as its name implies, is better suited for use in a dark environment.
In Tweetbot's settings, you can switch between default or night mode, as well as set it to automatically switch based on the brightness setting of your iOS device's screen.
There's another method for changing between the two modes that doesn't involve navigating through settings panes: Take two fingers, and swipe up or down on the screen while using Tweetbot 4. A swipe down enables night mode, while a swipe up reverts back to the default theme.
You'll need to leave Automatic switching between the two themes disabled in order for the gesture to work. Doing so gives you control over how the app looks based on your personal preference.