Signal-Android/app/src/main/res/drawable-v21/cta_button_background.xml

<?xml version="1.0" encoding="utf-8"?>
<ripple xmlns:android="http://schemas.android.com/apk/res/android"
    android:color="@color/core_ultramarine">

    <item android:id="@android:id/mask">
        <shape android:shape="rectangle">
            <corners android:radius="4dp" />
            <solid android:color="@color/core_ultramarine_dark" />
        </shape>
    </item>

    <item android:id="@android:id/background">
        <shape android:shape="rectangle">
            <corners android:radius="4dp" />
            <solid android:color="@color/core_ultramarine" />
        </shape>
    </item>

</ripple>