You can also try and provide an Id for the main layout and change the background of that through basic manipulation and retrieval. E.g:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/hello"
Which can then be followed by accessing through R.id.hello.... Pretty basic and I hope this does help :)