undefinedCSSClassesFinder.js

// Run this on browser console

var findUndefinedCSSClasses = (function () {
  "use strict";

  //get all unique CSS classes defined in the main document
  let allClasses = Array.from(document.querySelectorAll('*'))
    .map(n => Array.from(n.classList))
    .reduce((all, a) => all ? all.concat(a) : a)
    .reduce((all, i) => all.add(i), new Set());

  //load contents of all CSS stylesheets applied to the document
  let loadStyleSheets = Array.from(document.styleSheets)
    .map(s => {
      if (s.href) {
        return fetch(s.href)
          .then(r => r.text())
          .catch(e => {
            console.warn('Coudn\'t load ' + s.href + ' - skipping');
            return "";
          });
      }

      return s.ownerNode.innerText
    });

  Promise.all(loadStyleSheets).then(s => {
    let text = s.reduce((all, s) => all + s);

    //get a list of all CSS classes that are not mentioned in the stylesheets
    let undefinedClasses = Array.from(allClasses)
      .filter(c => {
        var rgx = new RegExp(escapeRegExp('.' + c) + '[^_a-zA-Z0-9-]');

        return !rgx.test(text);
      });

    if(undefinedClasses.length) {
        console.log('List of ' + undefinedClasses.length + ' undefined CSS classes: ', undefinedClasses);
    } else {
        console.log('All CSS classes are defined!');
    }
  });

  function escapeRegExp(str) {
    return str.replace(/[\-\[\]\/\{\}\(\)\*\+\?\.\\\^\$\|]/g, "\\$&");
  }

});

Example output on browser console: