index.css

* {
    box-sizing: border-box;
}

html, body {
    min-height: 100vh;
    margin: 0px;
    font-family: 'Roboto 500 ', sans-serif;
}

header {
    display: block;
    background-color: #427F99;
    padding: 1%;
    text-align: center;
    font-size: 150%;
    color: white;
}

nav {
    position: -webkit-sticky;
    position: sticky;
    top: 0;
    position: relative;
    float: left;
    width: 15%;
    height: 100%;
    background: #CCCCCC;
    padding: 1%;
}

nav ul {
    list-style-type: none;
    margin: 0;
    padding: 0%;
    font-size: 110%;
    font-weight: bold;
}

li a{
    margin: 0;
    display: block;
    color: #427F99;
    padding: 8px 16px;
    text-decoration: none
}

li a:hover {
    background-color: #427F99;
    color: white;
    font-size: 125%;
}

article {
    min-height: calc(100vh - 70px);
    min-height: 100%;
    text-align: justify;
    align-content: center;
    margin: auto;
    width: 85%;
    background-color: white;
    font-size: 120%;
}

section::after {
    content: "";
    display: table;
    clear: both;
}

footer {
    position: absolute;
    background-color: #427F99;
    width: 100%;
    padding: 20px;
    text-align: center;
    color: white;
}

div {
    margin: auto;
}

.big-header {
    margin-bottom:0;
    margin-top: 15px;
}

.little-header {
    margin-top : 0; 
    margin-bottom: 15px;
    padding-top:0;
    font-style: italic;
}

.sample {
    text-align: justify;
    padding-left: 25%;
    padding-right: 25%;
    margin: auto;
}