res/drawable/gutter_horizontal.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- This "gutter" has a shadow at the top and a subtler shadow on the bottom. -->
<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient android:type="linear"
              android:angle="-90"
              android:startColor="#ffE0E0E0"
              android:centerColor="#ffffffff"
              android:endColor="#ffF5F5F5" />
</shape>