MainActivity.java

 @Override
    protected void onCreate(Bundle savedInstanceState) {
        switch (getResources().getConfiguration().uiMode & Configuration.UI_MODE_NIGHT_MASK) {
            case Configuration.UI_MODE_NIGHT_YES:
                setTheme(R.style.DarkTheme);

                break;
            case Configuration.UI_MODE_NIGHT_NO:
                setTheme(R.style.LightTheme);
                break;
            default:
                setTheme(R.style.LightTheme);
        }

        SplashScreen.show(this, true);
        super.onCreate(savedInstanceState);
    }