Can't change Android app theme

I want to change the base theme of my application but every time I try, the application is terminating. I used to read the first comment from this question without any positive result. I'm attaching my AndroidManifest.xml and styles.xml.

Android.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://ift.tt/nIICcg"
package="com.fighttimer.stambeto09.fighttimer" >
<uses-sdk android:targetSdkVersion="15" android:minSdkVersion="7"/>
<uses-permission android:name="android.permission.VIBRATE"/>
<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".MainActivity"
        android:label="@string/app_name"
        android:screenOrientation="portrait">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

styles.xml

<style name="AppBaseTheme" parent="android:Theme.Holo.NoActionBar.Fullscreen">
</style>
<style name="AppTheme" parent="AppBaseTheme">

</style>