getClasses.js

var allStylesheets = document.styleSheets;
var classesUsed = [];
[].forEach.call(allStylesheets, function(sheet) {
    for (var i = 0; i < allStylesheets.length; i++) {
        try {
            var sheet = allStylesheets[i];
            for (var j = 0; j < sheet.cssRules.length; j++) {
                classesUsed.push(sheet.cssRules[j].selectorText);
            }
        } catch(e) {
            if (e.name !== "SecurityError") {
                throw e;
            }
        }
    }
});
console.log(classesUsed);