Instagram dark mode on iOS 13
Official method
In order to enable a global dark mode that will affect the Instagram app follow those steps:
1. Open Settings on your device
2. Go to Display and Brightness
3. Change the option to "Dark."
4. Enjoy Dark Mode on Instagram
Not official method
In order to enable a global dark mode that will affect the Instagram app follow those steps:
1. Settings
2. General
3. Accessibility
4. Accessibility Shortcut - It is located at the very bottom of the Accessibility screen.
5. Select Smart Invert Colours.
Now you can enabled and disable the iOS dark mode by clicking the home button three times. If you are using iPhone X, Xs, Xs Max or XR - click the new Slide button three times.
Voila, you can enjoy browsing the Instagram app with darker UI. Although it looks pretty good, one thing to consider is that videos come with inverted colors.
Here are few previews to decide wether it worth it for you or not