Q:

Toolbar doesn't work properly if I call setTheme into onCreate

I have a toolbar and 2 styles (one dark and one light) and I use SharedPreferences to select one, but toolbar not work properly.
Activity:
public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        SharedPreferences pref = PreferenceManager.getDefaultSharedPreferences(getApplicationContext());
        if (pref.getInt("DARK_MODE", 0) == 0) {
            setTheme(R.style.AppTheme_Light);
        } else {
            setTheme(R.style.AppTheme_Dark);
        }
        setContentView(R.layout.activity_main);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.main_menu, menu);
        return true;
    }
}

Toolbar:
<android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/toolbar"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:elevation="4dp"/>

Layout:
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <include layout="@layout/toolbar" />

</android.support.constraint.ConstraintLayout>

Style
<resources>

    <style name="AppTheme.Light" parent="@android:style/Theme.Material.Light">
        <item name="android:colorPrimary">@color/colorPrimary</item>
        <item name="android:colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="android:colorAccent">@color/colorAccent</item>
        <item name="android:titleTextColor">@color/white</item>           
    </style>

    <style name="AppTheme.Dark" parent="@android:style/Theme.Material">
        <item name="android:colorPrimary">@color/colorPrimary</item>
        <item name="android:colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="android:colorAccent">@color/colorAccent</item>
        <item name="android:actionMenuTextColor">@color/white</item>
    </style>

</resources>

The app shows me 2 toolbar, one with menu and one without. If i comment setTheme(…) call all work properly.

Why? How can I resolve this problem? 

A:

This is occurring because you are setting two toolbars:

One with the theme;
The other with setSupportActionBar(toolbar);

Use one or the other. In your case you should set the theme with no action bar Theme.AppCompat.Light.NoActionBar
You kind of answered the question yourself really "If i comment setTheme(…) call all works properly" :-D
The developer guides are always useful:
https://developer.android.com/training/appbar/setting-up

