SOURCE

<div class="wrapper">
  <div class="header"></div>
  <div class="content"></div>
</div>

.wrapper {
  background: red;
  height: 100vh;
}
.header {
  height: 50px;
  background: yellow;
}
.content {
  background: green;
  width: 100px;
  height: 100px;
  position: absolute;
  top: 50px;
  left: 0;
  right: 0;
  bottom: 0;
  margin: auto;
}