Custom icons in HADashboard

I’ve bought the Pro version of FontAwesome and I’m trying to get some icons uploaded for HADashboard.

I need to upload my all.css file to overwrite css/all.css but I can’t find the directory and file anywhere. find / -name all.css comes back with nothing but it’s there in the source and the free icons are working so it must be there somewhere!

I’m running Home Assistant with appdaemon as an add-on installed through Supervisor > Add-on store.

Anyone point me in the right place, please?