I'm new to css also and searched multiple forums. I found this solution by inspecting my squarespace webpage on google. It worked for me!
1.Copy and paste
@font-face { font-family: 'GeosansLight';
src: url('https://static1.squarespace.com/static/5a8c34eb51a584a57bc62511/t/5a93aadff9619a83af6f6839/1519626975231/GeosansLight.ttf ');
font-weight: normal;
font-style: normal; }
.has-site-title .Header-branding { font-family: "GeosansLight"; }
2.Changes you need to make for this to work--> a. upload your own font on 'manage custom file' and obtain 'src: url' by clicking on uploaded font file title. b. change 'font-family' and 'src: url'
