To set this theme for all the activites of your application, open the AndroidManifest.xml file and edit the <application> tag to include the android:theme attribute with the theme name.
If you want the theme applied to just one Activity in your application, then add the theme attribute to the <activity> tag, instead.
If you want to use system wallpaper as your own application's wall paper, add android:theme="@android:style/Theme.Wallpaper" in your AndroidManifest.xml.
Example:
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.exercise.AndroidSysTheme"
android:versionCode="1"
android:versionName="1.0">
<application android:icon="@drawable/icon" android:label="@string/app_name"
android:theme="@android:style/Theme.Wallpaper"
>
<activity android:name=".AndroidSysTheme"
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>
<uses-sdk android:minSdkVersion="5" />
</manifest>