<div class="parent">
  <div class="child">
  flexboxでど真ん中に
  </div>
</div>

.parent {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 200px;
  background: orange;
}

.child {
  display: block;
  width: 100px;
  background: #FFF;
}