res/drawable/gradient_actionbar_background.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (C) 2018 The Android Open Source Project

     Licensed under the Apache License, Version 2.0 (the "License");
     you may not use this file except in compliance with the License.
     You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

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     distributed under the License is distributed on an "AS IS" BASIS,
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<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient
        android:startColor="@color/tool_bar_gradient_max"
        android:endColor="@android:color/transparent"
        android:angle="270"
        android:type="linear" >
    </gradient>
</shape>