As you can see here, the new dark mode switches Instagram to a black background, making it easier on your eyes in low light conditions - which can have significant health benefits.
Dark mode has been one of the app's most requested features, but Instagram says that previous complications around implementing such within its coding have made it hesitant to enable the option. Some users have implemented their own workarounds to varying effect, but Instagram says that, due to improvements in iOS 13, it's now been able to make it an official reality.
Of course, that does mean you need to upgrade to the latest version of your respective operating system to use it, but it is now there, and available to try out.
Instagram has detailed the more technical elements of implementing dark mode over on the Instagram Engineering blog for those interested.
The addition means that dark mode options are now available on Messenger, Twitter, Pinterest and Instagram, while it's also in various stages of development for Facebook's main app. The options give your favorite apps a new look, while also, as noted, having health benefits.