With Android 10, Google finally introduced a system-wide dark mode to the platform. The new dark mode not only transforms the system UI but also lets you use supported apps in dark mode. Unfortunately, the toggle in Display settings is limited to devices running Android 10. In case you have a device running Android 9 Pie or older, however, there is a way to enable this same dark mode toggle, which apps that have added dark modes will respect.
You see, Android has had a hidden "night mode" option for a while now, but it first surfaced for the end-user with the release of Android 9 Pie. This night mode toggle, which was found in the Developer Options in Android 9, can also be toggled in older versions of Android via ADB or third-party apps. The night mode toggle is what apps listen to when deciding whether to change to their dark theme. Once it's turned on, the dark theme in apps like Google Photos, Play Store, Instagram, and more will be enabled even if you don't have a system-wide dark mode toggle in the standard Display settings.
In case you have a device running Android 8 Oreo or earlier, you can try it out for yourself by downloading one of the several third-party apps available on the Play Store. You can go for the Dark Mode app, which is designed just for this purpose and has a simple toggle that will help you choose between a Day Mode, a Night Mode, or an auto-switch setting. Or you can try our resident dev Zachary Wander's SystemUI Tuner app, which offers several other useful features as well. With these apps, you'll be able to enable a dark theme in a few apps on your old Android device without much effort. The Instagram light/dark mode screenshots above are from an LG G6 running Android 8 Oreo, which doesn't have any dark mode toggle anywhere in the Settings. Do note that this method won't work with all devices since some manufacturers completely disable the night mode option on their phones.
Via: Reddit