Dark mode / theme before Android 10? How is it possible (via third party app or without?) to enable Dark mode for all apps (Chrome browser, Phone calls app, Messaging app, etc.) on a Android 5 or 6 smartphone?

It's integrated in the OS in Android 10: https://developer.android.com/guide/topics/ui/look-and-feel/darktheme

But how to do this for Android 5+?
On Android 10, it's available as a system setting.

However the dark theme can be enabled using "night" resources for example. Meaning  switching resources such as styles, colours, themes, drawables, etc according to user's device time.


  This can change during the life of your app if night mode is left in auto mode (default), in which case the mode changes based on the time of day. You can enable or disable this mode using UiModeManager. See Handling Runtime Changes for information about how this affects your app during runtime.


You can force the mode or not using a toggle or something too. Read documentation for more info.