notification_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:background="#0f0"
              android:id="@+id/root"
              android:layout_width="match_parent"
              android:layout_height="match_parent">

    <Button xmlns:android="http://schemas.android.com/apk/res/android"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:background="#00f"
            android:padding="8dp"
            android:text="DEFAULT"
            android:textColor="#fff"
            android:textSize="20sp"/>

</LinearLayout>