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The issue is caused by a font-awesome style coming from either a modification in your theme or a plugin. To resolve this, add the following CSS.

If you’re not using a Child Theme, and if the theme doesn’t have a section for CSS modifications then do the following:

1. Install this Custom CSS Manager plugin https://www.ads-software.com/plugins/custom-css-manager-plugin
2. use its “CSS Code” section of the dashboard to hold your CSS modifications:
3. (put this code in)

   
   html .icon {
       display: inline-block;
       fill: currentColor;
       height: 1em;
       position: relative;
       top: -0.0625em;
       vertical-align: middle;
       width: 1em;
   }
   

4. Save

Alternatively use your Child Theme style.css file to hold your CSS modifications

Just a heads-up that WordPress 4.7 now includes a custom CSS panel in the Customizer — so if you’re not already using a child theme, you can use the new panel without needing to install a separate plugin. ??

Thanks Kathryn,
I’ll probably still use that boilerplate answer because I want people to choose themselves which method to use. If I start recommending to use the Custom CSS panel when people have already set up a Child Theme they might find using both methods unmanageable, likewise if they were already using a Custom CSS plugin.

Many thanks @anevins and @zoonini. I inserted the code into the ‘Additional CSS’ and it works a treat.