Android - How to detect if night mode is on when using AppCompatDelegate.MODE_NIGHT_AUTO

int nightModeFlags =
    getContext().getResources().getConfiguration().uiMode &
    Configuration.UI_MODE_NIGHT_MASK;
switch (nightModeFlags) {
    case Configuration.UI_MODE_NIGHT_YES:
         doStuff();
         break;

    case Configuration.UI_MODE_NIGHT_NO:
         doStuff();
         break;

    case Configuration.UI_MODE_NIGHT_UNDEFINED:
         doStuff();
         break;
}

If you are kotlin developer then you can use below code to judge dark mode.

 val mode = context?.resources?.configuration?.uiMode?.and(Configuration.UI_MODE_NIGHT_MASK)
    when (mode) {
        Configuration.UI_MODE_NIGHT_YES -> {}
        Configuration.UI_MODE_NIGHT_NO -> {}
        Configuration.UI_MODE_NIGHT_UNDEFINED -> {}
    }

For more about dark theme mode

https://github.com/android/user-interface-samples/tree/main/DarkTheme

The bitwise operator in Java (which is used in @ephemient 's answer) is different in kotlin so the code might not be easily convertible for beginners. Here is the kotlin version just in case someone needs it:

    private fun isUsingNightModeResources(): Boolean {
        return when (resources.configuration.uiMode and 
Configuration.UI_MODE_NIGHT_MASK) {
            Configuration.UI_MODE_NIGHT_YES -> true
            Configuration.UI_MODE_NIGHT_NO -> false
            Configuration.UI_MODE_NIGHT_UNDEFINED -> false
            else -> false
    }
}