Make sure you've updated your Instagram app and are using the latest available version.
One thing that you should note is, that the dark mode theme on Instagram works in correspondence with the system theme you are using on your iPhone. Once reports started coming that it is being tested in Instagram Alpha, it arrived to the stable app in only two weeks. If you're not on these two platforms you can not experience the new Dark Mode.
Instagram has joined the dark side. Dark mode is now available for Facebook Messenger's app. On some displays, black pixels require no power at all. When your phone goes dark, so too will the popular social media app.
Google actually started a real revolution with a dark theme earlier this year.
Tap Advanced, then select Dark from the Device theme menu. It updated a ton of its applications thus far, and those that were not updated before Android 10's arrival, did get the update soon after. Microsoft revealed today that the dark modes, which were announced earlier this summer, are now rolling out for Office Insiders testing Word, PowerPoint, Excel, and OneNote on iOS 13. Facebook's Messenger app also adopts a true dark theme, with a proper black background.