how to change background to light grey

I’m glad you find the plugin useful!

As described in the FAQ, this plugin comes with a starter CSS file to help you customize the widget output. These can be copied into a child theme stylesheet or used in the Customize > Custom CSS setting.

While custom CSS changes are outside the scope of this support forum, you probably are looking for something like this:

.widget_fpw_widget {
    background-color: #ccc;
    padding: 1em;
}