styles.less

atom-text-editor::shadow {
    .source {
        font-family: FiraCode-Retina;
        text-rendering: optimizeLegibility;
    }

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

    .source.js.jsx > .keyword.control.flow.js,
    .storage:not(.arrow), // allow "Fira Code" to show arrow ligature
    .comment,
    .type .function,
    // .keyword.control, // a bit too heavy-handed...
    .entity.other.attribute-name.js {
        font-family: 'Space Mono'; // https://fonts.google.com/specimen/Space+Mono
        font-style: italic;
        line-height: 1;
        letter-spacing: -0.01em; // match "Fira Code" char width
    }
}