simplauncher/app/src/main/res/drawable/app_background.xml

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

    <corners
        android:radius="8dp" />

    <stroke android:color="?android:colorForeground"
        android:width="1dp"/>

</shape>