JSFiddle - React, Tailwind, and code Playground

HTML

<div class="menu">
    <div id="menu-center">
        <ul>
            <li><a class="active" href="#home">Home</a>
            </li>
            <li><a class="inactive" href="#portfolio">Portfolio</a>
            </li>
            <li><a class="inactive" href="#about">About</a>
            </li>
            <li><a class="inactive" href="#contact">Contact</a>
            </li>
        </ul>
    </div>
</div>
<div id="home"></div>
<div id="portfolio"></div>
<div id="about"></div>
<div id="contact"></div>

CSS

body, html {
    margin: 0;
    padding: 0;
    height: 100%;
    width: 100%;
}
.menu {
    width: 100%;
    height: 75px;
    background-color: rgba(0, 0, 0, 1);
    position: fixed;
     transition: all 1000ms ease-in;
}
.light-menu {
    width: 100%;
    height: 75px;
    background-color: rgba(255, 255, 255, 1);
    position: fixed;
     transition: all 1000ms ease-in;
}
#menu-center {
    width: 980px;
    height: 75px;
    margin: 0 auto;
}
#menu-center ul {
    margin: 15px 0 0 0;
}
#menu-center ul li {
    list-style: none;
    margin: 0 30px 0 0;
    display: inline;
}
.active {
    font-family:'Droid Sans', serif;
    font-size: 14px;
    color: #fff;
    text-decoration: none;
    line-height: 50px;
}
.inactive {
    font-family:'Droid Sans', serif;
    font-size: 14px;
    color: #9e9e9e;
    text-decoration: none;
    line-height: 50px;
}
#home {
    background-color: grey;
    height: 100%;
    width: 100%;
    overflow: hidden;
    background-image: url(images/home-bg2.png);
}
#portfolio {
    background-image: url(images/portfolio-bg.png);
    height: 100%;
    width: 100%;
}
#about {
    background-color: blue;
    height: 100%;
    width: 100%;
}
#contact {
    background-color: red;
    height: 100%;
    width: 100%;
}

JavaScript

$(document).ready(function () {

    var menu = $('.menu');

    $(window).scroll(function () {

        var y = $(this).scrollTop();
        var z = $('#portfolio').offset().top;

        if (y >= z) {
            menu.removeClass('menu').addClass('light-menu');
        }

    });

});