body {
    background-color: #eee;
    font-size: 18px;
    width: min(700px, 80%);
    margin: auto;
}

h1, .subtitle {
    text-align: center;
}

img {
    display: block;
    margin: auto;
}

div.sourceCode, pre:not(.sourceCode) {
    background-color: #fff;
    padding: 10px 15px;
    width: 90%;
    margin: auto;
    border: 1px solid #666;
}