Android Set Button Background Transparent

        <Button
            android:id="@+id/buttonSaveHeight"
            android:layout_marginTop="20dp"
            android:layout_marginBottom="10dp"
            android:text="@string/ok"
            android:gravity="center"
            android:layout_gravity="center"
            android:background="@android:color/transparent"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" />

References
https://github.com/mhdr/AndroidSamples/tree/master/074