nav-bar.css

body {
    margin: 0;
}

body>div {
    margin: 10px;
}

nav {
    list-style-type: none;
    margin: 0;
    padding: 0;
    overflow: auto;
    background-color: gray;
    position: sticky;
    top: 0px;
    z-index: 0;
}

nav>a {
    float: left;
    display: block;
    color: white;
    text-align: center;
    padding: 14px 16px;
    text-decoration: none;
    cursor: pointer;
}

nav>a:hover:not(.active) {
    background-color: black;
}

nav>a.active {
    background-color: purple;
}