There’s two things going on here.
Dark Mode is a new, long-awaited feature of iOS 13. It provided a darker look to the interface with apps going with a fully black background. I like it better for my own eyes, especially in dimmer environments. Also, for people like me with OLED displays, those blacks are true blacks, meaning the pixels are actually off where true black is shown. That can save some battery life in the long run.
The most recent beta of iOS 13 offers Dark Mode as an accessibility shortcut, which is activated by a triple-click of the power button. It’s a bit quicker than digging through control center where the dark mode setting is buried. That allows me to demonstrate just how dark Dark Mode makes a wallpaper when activated.