Set background of your app to be transparent

To set background of your app to be transparent, you can use Android pre-defined Theme.Translucent.

Modify AndroidManifest.xml, to add the code inside <activity>
android:theme="@android:style/Theme.Translucent"

example:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.AndroidTranslucent"
    android:versionCode="1"
    android:versionName="1.0">
  <uses-sdk android:minSdkVersion="4" />

  <application android:icon="@drawable/icon" android:label="@string/app_name">
      <activity android:name=".AndroidTranslucent"
                android:label="@string/app_name"
                android:theme="@android:style/Theme.Translucent">
          <intent-filter>
              <action android:name="android.intent.action.MAIN" />
              <category android:name="android.intent.category.LAUNCHER" />
          </intent-filter>
      </activity>

  </application>
</manifest>