How do I access font files I have saved in the custom css section?

Site URL: https://buttercup-swan-mjg6.squarespace.com/config/design/custom-css

Hi there, 

Any help and guidance here would be much appreciated. I am trying to use 4 different font files throughout the site - I have added them to the custom css section however when I access the font area to select them they are not appearing in the list. can anyone advise please... 

I have attached two screenshots showing what I am seeing the custom css area and I am aware there is an error message, but this file is correct so I am confused what I am doing wrong. 

Again help would be much appreciated. 

Do you mean when you go to the squarespace site styles editor the fonts aren't appearing as an option? Or that the fonts aren't appearing in your css files? It looks like the files are all there (you just need to scroll in the second screenshot). They won't appear in the site styles editor, you have to put in css to code the fonts into the site.