AppCompate Themes doesn't view day or night view

Android Programming (4th Edition) 21. Styles and Themes
1

if I change the switch: MODE_NIGHT_YES to NO, nothing is changing in the app
I cannot find a sampe wirh the java Nerd 4 construction.
An InitApplication exist in the directory, but where should I implement folloeing lines to call the InitApplication?
The sample is based on a AppCompatActivity, but I have

Sample Code

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if (InitApplication.getInstance().isNightModeEnabled()) {
        AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_YES);
    } else {
        AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_NO);
    }
    setContentView(R.layout.activity_main);


**MainAcitivity**
    public class Activity_MindX extends SingleFragmentActivity
    {
	private final String LOG = this.getClass().getName();
	
    @Override
    protected Fragment createFragment() {
        return new FragmentMenuMindX();
    }
	// ################################################
    public void onCardSetUpdated(CardSet cardset) {
    // ################################################
    	FragmentManager fm = getSupportFragmentManager();
    	fm.findFragmentById(R.id.fragmentContainer);
    	}

in the Fragment

    public class FragmentMenuMindX extends Fragment implements OnClickListener,
		OnChartValueSelectedListener,
		SeekBar.OnSeekBarChangeListener,
		MindXconstants, MessageConst, DialogConstants {
    }

int the FragemtActivity

    @TargetApi(11)
	@Override
	// ################################################
	public View onCreateView(LayoutInflater inflater, ViewGroup parent,
			Bundle savedInstanceState) {
	// ################################################
		**AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_YES);**
		View v = inflater.inflate(R.layout.activity_menu_mind_x, parent, false