index.html

<div class="container">
  <div class="d-flex flex-column justify-content-center main" >
      <div class="blue">
          <h3 class ="text-center">Blue Box</h3>
      </div>
  </div>
</div>

style.css

.blue{
  background-color :#3498db;
  width: 100%;
  height :100%;

}

.main{
  height :100vh;
  border: 2px solid #2c3e50;
}