Does this really old forum post still work?
mark (Retool Team) Jan '21 For anyone who may be looking for this in the future, as an alternative, you are now able to add the following line to your “Preloaded CSS” under Advanced Settings, to use a CDN hosted version of FontAwesome 5.
@import url(https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.15.2/css/all.min.css)