This is the stylesheet I use in Atom to get the design I like. It requires Fira Code (mono font) and FlottFlott (ligature font). But you can swap out your own fonts by replacing them in the sheet.

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
 */


/*
 * Examples
 * (To see them, uncomment and save)
 */

// style the background color of the tree view
.tree-view {
  // background-color: whitesmoke;
}

// style the background and foreground colors on the atom-text-editor-element itself
atom-text-editor {
  // color: white;
  // background-color: hsl(180, 24%, 12%);
}

// To style other content in the text editor's shadow DOM, use the ::shadow expression
// atom-text-editor::shadow .cursor {
//   // border-color: red;
// }

// Adding ligatures

atom-text-editor {
  font-family: 'Fira Code';
  font-style: normal;
  text-rendering: optimizeLegibility;
}
atom-text-editor.editor {
  .syntax--string.syntax--quoted,
  .syntax--string.syntax--regexp {
    -webkit-font-feature-settings: "liga" off, "calt" off;
  }
  .syntax--source.syntax--js.syntax--jsx > .syntax--keyword.syntax--control.syntax--flow.syntax--js,
  .syntax--storage, .syntax--type .syntax--function {
    vertical-align: baseline;
    font-family: 'flottflott';
    height: inherit;
    font-size: 1.5em;
    line-height: 1rem;
  }
  .syntax--source.syntax--js.syntax--jsx,
  .syntax--storage.syntax--type.syntax--function.syntax--arrow.syntax--js,
  .syntax--variable, .syntax--support.syntax--function.syntax--construct {
    font-family: 'Fira Code';
    font-style: normal;
  }
  .syntax--string.syntax--unquoted.syntax--js {
    color: #CDD3DE;
  }
  .syntax--entity.syntax--name {
    font-weight: bold;
  }
  .syntax--review {
    font-family: 'Fira Code';
  }
  .syntax--keyword.syntax--control:not(.syntax--regexp) {
    vertical-align: baseline;
    font-family: 'flottflott';
    height: inherit;
    font-size: 1.5em;
    line-height: 1rem;
  }
  .syntax--storage.syntax--type.syntax--class {
    &.syntax--note, &.syntax--todo, &.syntax--review {
      vertical-align: baseline;
      font-family: 'Fira Code';
      height: inherit;
      font-weight: 600;
      font-size: 1.3em;
      line-height: 1rem;
    }
  }
}