Operator Mono with Fira Code ligatures in Atom

styles.less

/*
 * Your Stylesheet
 *
 * This stylesheet is loaded when Atom starts up and is reloaded automatically
 * when it is changed and saved.
 *
 * Add your own CSS or Less to fully customize Atom.
 * If you are unfamiliar with Less, you can read more about it here:
 * http://lesscss.org
 */

atom-workspace,
atom-text-editor {
  font-family: "Operator Mono SSm";
  font-weight: 400;
  line-height: 1.7;
}

atom-text-editor.editor {
  .syntax--storage.syntax--type.syntax--function.syntax--arrow,
  .syntax--keyword.syntax--operator:not(.accessor),
  .syntax--punctuation.syntax--definition {
    font-family: "Fira Code";
  }

  .syntax--string.syntax--quoted,
  .syntax--string.syntax--regexp {
    -webkit-font-feature-settings: "liga" off, "calt" off;
  }
}