This is because in the latest update to the app, Instagram has introduced dark mode to the platform. This means that a flip of a switch (or toggle), the app will take on a dark look, as you can see in the screenshot above. The update will give the app a black and grey look that should make it easier on your eyes.
Dark mode is useful especially at night when you might want to reduce the glare from Instagram’s otherwise white and bright interface. It has been suggested that dark mode could even help save battery on AMOLED devices due to how black in AMOLED basically means that the pixels are turned off, thus potentially saving battery.
The update to Instagram should already be live so if you have yet to update the app, you might want to do so if you want to take advantage of its new dark mode setting.