I set the color to white but it won't change: activity_my.xml:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context=".MyActivity"
    android:theme="@style/AppTheme">
</RelativeLayout>
style.xml
<resources>
    <!-- Base application theme. -->
    <style name="AppTheme" parent="android:Theme.Holo.Light.DarkActionBar">
        <item name="android:actionBarStyle">@style/ActionBar</item>
    </style>
    <style name="ActionBar">
        <item name="android:background">#ffffff</item>
    </style>
</resources>
androidmanifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="eu.test.test.test" >
    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".MyActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>
I've searched, but everywhere is explained this way.
I've added the androidmanifest file.
 
     
    