Dark (or night) mode has become a must have feature for apps and devices over the past year or so. It is basically a display theme that is easier on the eyes in low light settings. A typical dark mode has some variation of a darker background with lighter text as opposed to the normal white background/black text, making it much more comfortable to read your phone in the dark.
Apple has consistently added features and content to its stock reader app. Originally known as iBooks, on iOS 12 and later the app has been renamed Books. iPhone, iPad and iPod touch users may notice that when reading, Books automatically enters a night theme when the room is dark. This Dark Mode can be manually controlled and customized directly in the Books app.
While Apple has yet to offer an official "Dark Mode" setting for low light environments, iOS 11 does offer "Smart Invert Colors" in the Accessibility settings, which basically turns white backgrounds to black and black text to white (and vice-versa) and dims bright colors. This makes viewing in low light situations easier on the eyes and less distracting to others around you. Unfortunately it is not very easy to get to in a hurry because it is buried in Settings -> General -> Accessibility -> Display Accommodations -> Invert Colors -> Smart Invert.
iOS fans have been clamoring for a "dark mode" feature for a long time now. A true dark mode feature would change the colors and brightness of certain areas of the screen, making it more suitable for dark environments like movie theaters, where a bright screen is liable to annoy fellow movie goers. This is different than Night Shift, which reduces eye stress caused by blue light by shifting the display to the red end of the spectrum.
Apple introduced a more theatrical option for Apple TV owners in tvOS 10 called Dark Mode. Instead of the normal light grey interface, this provides a different flavor for night viewing or those who prefer this look. The color change affects tvOS throughout the system menus and within particular apps. Currently there is no way to make Dark Mode switch on automatically at sunset or a specific time.