@android:style/Theme.NoTitleBar.Fullscreen doesn't force the whole app to fullscreen

These are the first lines of my AndroidManifest.xml:

----------------------------------------------------------------------------
<?xml version='1.0' encoding='utf-8'?>
<manifest android:installLocation="auto" android:versionCode="1" package="org.qtproject.example" xmlns:android="http://schemas.android.com/apk/res/android" android:versionName="1.1">
<application android:theme="@android:style/Theme.NoTitleBar.Fullscreen" android:label="PokerTH" android:name="org.qtproject.qt5.android.bindings.QtApplication" android:icon="@drawable/icon">
<activity android:label="PokerTH" android:configChanges="orientation|uiMode|screenLayout|screenSize|smallestScreenSize|locale|fontScale|keyboard|keyboardHidden|navigation" android:name="org.qtproject.qt5.android.bindings.QtActivity" android:screenOrientation="landscape">
----------------------------------------------------------------------------

I want to have all dialogs fullscreen that's why i set @android:style/Theme.NoTitleBar.Fullscreen.
I guess this should force the whole application to be shown in fullscreen.

I can do this->showFullscreen() in every single dialog implementation class which works, but i thought the above way via Manifest is the intended way of doing that once for the whole app.