[notsure]

Sure!

This is what I put into calibre's extra css section;

@font-face {
  font-family: "Fontin";
  font-weight: normal;
  font-style: normal;
  src: url(res:///Data/fonts/Fontin-Regular.ttf);
}

@font-face {
  font-family: "Fontin";
  font-weight: bold;
  font-style: normal;
  src: url(res:///Data/fonts/Fontin-Bold.ttf);
}

@font-face {
  font-family: "Fontin";
  font-weight: normal;
  font-style: italic;
  src: url(res:///Data/fonts/Fontin-Italic.ttf);
}

body {
  font-family: "Fontin", serif;
}