Monkey patch for jQuery css to handle custom properties aka css variables

css.js

$.fn.css = (function(css) {
    return function fn(key, value) {
        var self = this;
        if (arguments.length == 1 && $.isPlainObject(arguments[0])) {
            var data = arguments[0];
            Object.keys(data).forEach(function(key) {
                fn.call(self, key, data[key]);
            });
        } else if (key.match(/^\s*--/)) {
            if (typeof value === 'undefined') {
                return self[0].style.getPropertyValue(key);
            } else {
                self[0].style.setProperty(key, value);
            }
        } else {
            return css.apply(self, arguments);
        }
        return self;
    };
})($.fn.css);