android/sample/res/drawable/action_bar_background.xml

<?xml version="1.0" encoding="utf-8" ?>

<!--
  Copyright 2014-present, Facebook, Inc.
  All rights reserved.

  This source code is licensed under the license found in the
  LICENSE-examples file in the root directory of this source tree.
-->

<shape xmlns:android="http://schemas.android.com/apk/res/android"
  android:shape="rectangle"
  >
  <solid
    android:color="@color/yoga_grey"
    />
</shape>