.box {
  display: flex;
  align-items: center;
  justify-content: center;
}

.box div {
  width: 100px;
  height: 100px;
}

<div class="box">
  <div></div>
</div> 
https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout/Aligning_Items_in_a_Flex_Container

อื่นๆ
https://www.babelcoder.com/blog/articles/flexbox-and-auto-margins
https://stackoverflow.com/questions/19026884/flexbox-center-horizontally-and-vertically