One of iOS 13’s major new features is a systemwide Dark Mode option, which matches the Dark Mode feature we got in macOS Mojave last year.
Enabling Dark Mode can be done through the Settings app on the iPhone (or iPad) under the Display & Brightness section. You can select Light or Dark mode, or choose to enable them based on the time of day (Sunrise to Sunset) or a custom-picked schedule. If you long press on the Brightness indicator in Control Center, you can toggle on Dark Mode from there if you don’t want to use the Settings app, and there’s also a full Dark Mode/Light Mode toggle you can enable.
To make sure you have that Dark Mode Control Center option, go to Settings > Control Center > Customize Controls. From there, tap the green “+” next to “Dark Appearance” to add that button to Control Center.