android-studio how to set xml theme

I am using android-studio..

I am creating activity.xml file..There from the themes I choose a Light.NoTitlebar in graphical view.

And then when I run that app..It is showing the default which is in styles.xml

 <style name="AppTheme" parent="android:Theme.Holo">
        <!-- Customize your theme here. -->
    </style>

And my Manifestfile.xml is

 <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >

How can I change the xml theme whatever I want??