session-android/res/drawable/default_session_background.xml

<?xml version="1.0" encoding="utf-8"?>
<shape
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle" >

    <gradient
        android:angle="90"
        android:startColor="@color/default_background_start"
        android:endColor="@color/default_background_end"
        android:type="linear" />

</shape>