How to make spinner background transparent in android?

Here is my spinner layout,

<Spinner
    android:layout_marginLeft="20dp"
    android:background="@android:drawable/btn_dropdown"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:id="@+id/spinnerQuality"
    android:layout_gravity="center_horizontal"
    />

and spinner item layout,

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://ift.tt/nIICcg"
    android:id="@android:id/text1"
    android:singleLine="true"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:textColor="#000000"
    android:paddingTop="10dp"
    android:paddingLeft="10dp"/>

and showing spinner is,

But i want to get both spinner and dropdown items background transparent. How would i get that?