Question

I have a background

is not a simple color I do not know how to set the background of your application

Answer 1

So if you want to display image in background and repeat it put this code in drawable folder:
bg.xml

<bitmap xmlns:android="http://schemas.android.com/apk/res/android"
    android:src="@drawable/your_image"
    android:tileMode="repeat" />

and then simply apply this to your root view:
android:background="@drawable/bg"