While the Mac received an official dark mode option in macOS Mojave, iOS wasn’t so fortunate. You can use the Smart Invert feature on your iPhone to achieve a similar effect, but it’s not a real dark mode in the sense of what many would like to have.
Luckily, the jailbreak community conceived a plethora of dark mode-centric jailbreak tweaks in recent memory that can help, and a new free release called DarkTables by iOS developer dpkg_ is no exception.