Edit in JSFiddle

<div class="conteneur">
    <div class="contenu">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Molestias libero magni beatae corporis repellat ullam nobis voluptatibus ipsum! Aut voluptate excepturi cum praesentium obcaecati quos quas ipsum nisi repellendus aliquid?</div>
</div>

.conteneur {
    width: 550px;
    height: 150px;
    margin: auto;
    position: relative;
    background: #3498db;
    color: white;
}

.contenu {
    position: absolute;
    width: 85%;
    height: 85%;
    margin: auto;
    left: 0;
    right:0;
    top: 0;
    bottom: 0;
    background: #2980b9;
}