Signal-Android/app/src/main/res/drawable/story_gradient_bottom.xml

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

    <gradient android:type="linear"
        android:angle="90"
        android:startColor="@color/story_caption_gradient_start" />

</shape>