There wasn't any good way of loading Basscss with all of its addons (via lein). This, when injected into index.html, gets the job done.

load-basscss.js

(() => {
  var basscssAddons = [
    'responsive-margin', 'responsive-padding', 'forms', 'btn',
    'btn-outline', 'btn-primary', 'btn-sizes', 'colors',
    'background-colors', 'background-images', 'border-colors', 'darken',
    'lighten', 'input-range', 'progress', 'all', 'media-object',
    'highlight', 'highlight-dark',
  ];

  function createBasscssAddonLink(addon) {
    if (addon === 'forms') {
      return 'https://npmcdn.com/basscss-forms/index.css';
    }
    return 'https://npmcdn.com/basscss-' + addon + '/css/' + addon + '.css';
  }

  function fetchCSS(link) {
    return fetch(link).then((response) => response.text());
  }

  function createDOMElement(name, props) {
    return Object.assign(document.createElement(name), props);
  }

  function createStyleElement(css) {
    return createDOMElement('style', {
      type: 'text/css',
      innerHTML: css,
    });
  }

  function fetchBasscssWithAddons() {
    var links = [
      'https://npmcdn.com/basscss/css/basscss.min.css',
      ...basscssAddons.map(createBasscssAddonLink)
    ];
    return Promise
      .all(links.map(fetchCSS))
      .then((css) => css.join('\n'));
  }

  function injectCSS(css) {
    document.head.appendChild(createStyleElement(css));
  }

  document.addEventListener('DOMContentLoaded', () => {
    fetchBasscssWithAddons()
      .then(injectCSS)
      .then(
        () => console.log('Loaded Basscss!'),
        (error) => console.error('Failed to load Basscss!', error)
      );
  });
})();