Welcome Activity with background of image

I use the theme to show my WelcomeActivity

@android:style/Theme.NoTitleBar.Fullscreen

But I found it will have a period of black background when the app start

So I want to change the background of this theme

then I use...

<style name="WelcomeTheme" parent="@android:style/Theme.NoTitleBar.Fullscreen">
    <item name="android:windowBackground">@drawable/welcome_img</item>
    <item name="android:windowNoTitle">true</item>
    <item name="android:windowActionBar">false</item>
</style>

Although the background is changed , the Actionbar is also showing!!

the Actionbar doesn't show when I use Theme.NoTitleBar.Fullscreen

but it's showing when I use WelcomeTheme

how should I remove the ActionBar

Thanks for your help :)

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By the way , in my WelcomeActivity extends Activity but not ActionBarActivity