Add a DarkMode Button to your activity

AddDarkModeButton.kt

fun addDarkModeButton() {
    val darkButton = Button(this).apply {
      text = if (getDefaultNightMode() != MODE_NIGHT_YES) "Go Dark " else "Go Light"
      layoutParams = LayoutParams(MATCH_PARENT, WRAP_CONTENT)
    }
    
    (window.decorView as ViewGroup).addView(darkButton) // or `yourContainer.addView(darkButton)`

    darkButton.setOnClickListener {
      AppCompatDelegate.setDefaultNightMode(
        if (getDefaultNightMode() != MODE_NIGHT_YES) MODE_NIGHT_YES else MODE_NIGHT_NO
      )
    }
  }