Edit in JSFiddle

<div class="flex-center">
  <p>Nous sommes plusieurs lignes centrées verticalement dans un container de type flex.</p>
</div>

body {
  background: #3e58aa;
  font-size: 80%;
}

div {
  background: white;
  width: 240px;
  margin: 20px;
}

.flex-center {
  background: #CCDCCD;
  color: #3e58aa;
  border: 10px solid white;
  display: flex;
  flex-direction: column;
  justify-content: center;
  height: 200px;
  resize: vertical;
  overflow: auto;
}
.flex-center p {
  margin: 0;
  padding: 20px;
}