<widget-icon> actually has a backdrop boolean attribute which will set up the correct CSS classes for you 
So you simply configure an regular Icon setting, give it a name e.g. ‘backdrop’ then:
<widget-icon iconset="config.backdrop_iconset" icon="config.backdrop"
backdrop="true" center="true"></widget-icon>
<div>
<h2>Hi!</h2>
<p>This should go over the backdrop</p>
</div>
Will render as:
