Can I simply add directories to my Discourse install?

Development
2

I’m not sure about adding directories, but regarding using font files in themes, there is a built in method you could try. If you’re editing the Theme files manually, you can add the font to the theme’s /assets directory, then add an entry for it in about.json like this:

{
  // other properties
  "assets" : {
    "my-font-identifier": "assets/my-font-file.woff2",
  }
}

Then in the @font-face you can reference it with src: url($my-font-indentifier);

If you’re just using the Theme editor in the settings interface, I believe you can add it to the uploads section, put the identifier you want to use in the SCSS var name field, then reference it the same way in the SCSS.