Reddit App
There are two ways to turn on dark mode in the Reddit app. The easiest way is to tap your user profile in the upper-left corner and select the moon icon in the bottom of the pullout menu. This will set the theme to black.
Reddit also offers several different color scheme choices, and dark mode can be found there as well. If you want to take the long way around, tap on Settings > Theme to choose between Alien Blue, Mint, Night, Pony, and Trees color schemes.
There are other theme settings that can help optimize your dark mode experience. You can also select Auto Night Mode to change things based on time of day, and AMOLED Night Mode, which removes all accent colors. A dark theme is also available for the web version by clicking your username and toggling Night Mode to on.