Atom font-style settings found in Atom.io's configuration settings.

styles.less

// ==== Ligature fonts for atom
//      JM 12/14/2016
// Sources:
//  - https://github.com/tonsky/FiraCode
//  - http://www.dafont.com/flottflott.font
//
// Reference (original): https://medium.com/@docodemore/an-alternative-to-operator-mono-font-6e5d040e1c7e#.nm9fchwv1
// Reference (improved): https://gist.github.com/MattMcFarland/e41ef709b1d82adea800563a86805559#gistcomment-1835618

@fontsize: 16px;

atom-text-editor {
  font-family: 'Fira Code';
  font-style: normal;
  text-rendering: optimizeLegibility;
  // letter-spacing: 1.45px;

  &::shadow {
    .string.quoted,
    .string.regexp {
      -webkit-font-feature-settings: "liga" off, "calt" off;
    }
    .source.js.jsx > .keyword.control.flow.js,
    .storage, .type .function {
      vertical-align: baseline;
      font-family: 'flottflott';
      height: inherit;
      font-size: 1.5 * @fontsize;
      line-height: @fontsize;
    }
    .source.js.jsx,
    .storage.type.function.arrow.js,
    .indent-guide,
    .variable {
      font-family: 'Fira Code';
      font-style: normal;
      font-size: @fontsize;
      line-height: 1.5 * @fontsize;
    }
    .string.unquoted.js {
      color: #CDD3DE;
    }
    .entity.name {
      font-weight: 500;
    }
  }
}