body {
    margin: 0;
    font-family: Verdana, Geneva, Tahoma, sans-serif;
}

ul {
    list-style-type: none;
    margin: 0;
    padding: 0;
    width: 25%;
    background-color: #dddddd;
    position: fixed;
    height: 100%;
    overflow: auto;
}

li a {
    display: block;
    color: #000;
    padding: 8px 16px;
    text-decoration: none;
}

li a.activo {
    background-color: #949e05;
    color: white;
}

li a:hover:not(.activo) {
    background-color: #555;
    color: white;
}


.contenedor {
    margin-left: 25%;
    padding: 1px 16px;
    height: 1000px;
}