<div class="box">
  <div class="row header"><p>Header (hauteur fixe)</p></div>
  <div class="row content">
    <p>
      Content (forcer ce div à utiliser l’espace restant)
    </p>
  </div>
  <div class="row footer"><p>Footer (hauteur fixe)</p></div>
</div>

html,
body {
  height: 100%;
  margin: 0;
}

.box {
  display: flex;
  flex-flow: column;
  height: 100%;
}

.box .row {
  border: 1px dotted grey;
}

.box .row.header {
  flex: 0 1 35px;
  background:lightpink;
}

.box .row.content {
  flex: 1 1 auto;
  background:lightblue;
}

.box .row.footer {
  flex: 0 1 35px;
  background:lightpink;
}