I can't change the notification icons

Development
7

May I add this for future reference. I followed the instructions on Replace Discourse's default SVG icons with custom icons in a theme - #19 but but what I did not realise at first was that the same icon can be referred to in different ways. In the app itself, under ‘/styleguide/atoms/icons’ is a list of all the icons and how they are referred to, so in my header.html file I now have 5 references to the envelope icon:

    api.replaceIcon('envelope', 'fgx-air-mail');
    api.replaceIcon('far-envelope', 'fgx-air-mail');
    api.replaceIcon('notification.private_message', 'fgx-air-mail');
    api.replaceIcon('notification.read_private_message', 'fgx-air-mail');
    api.replaceIcon('notification.invited_to_private_message', 'fgx-air-mail');