With the dashboard button icons can be selected. That selection just shows the icon name like 'fa-refresh'. How to see more than the name? It's a bit time consuming to add and commit to see how it looks.
The dir ~/.node-red/node_modules/node-red-dashboard/dist/fonts seems to contain also icons, but how to handle files with .woff /-woff2?
Yes the icon names start with 'fa-'
The page you mention has many many icons, but only 'fa-spin' is referenced on it.
Would any named icon on that page available as a dashboard icon?