The official Instagram help center says that only devices with Android 10 and iOS 13 can turn on the dark mode. However, for older smartphones, there are also ways to turn on the dark theme, but they are not so simple.
How to turn on dark mode on Android.
On Android, dark mode is activated in the settings. Scroll down to the Screen menu and turn on the Dark Theme button. Unfortunately, this setting is not available on all devices. Therefore, if you have just such a smartphone, then you can use third-party applications from Google Play.
Please note that a dark theme activated through the application may work partially or incorrectly, and is also not applicable to all devices. You may need to try several different applications. If successful, Instagram will also go into dark mode automatically.
How to turn on dark mode on iOS.
Like Android, the dark mode for Apple devices is turned on along with the system settings. Such a function is available on iOS 13. To turn on dark mode, go to Settings and select “Screen and Brightness”, then activate the dark theme.
It can also be turned on from the Control Center. Swipe the screen up and hold your finger on the brightness control. In the window that opens, quick switching of the dark and light modes will be available.
How to turn on dark mode on the old version of iO42S.
On earlier versions - iOS 11 and 12, you can use smart inversion. This is not a full-fledged, but still a replacement for the dark theme. Smart inversion is a kind of predecessor of a full-fledged dark theme, so it works the same way. Go to the settings and select "Universal Access", then "Display and text size." Turn on Smart Inversion.