How do I center an item vertically and horizontally in a grid container?

<!-- BEGIN COPY / PASTE -->
    <div class="grid-container">
      <div class="grid-item">Centered Item</div>
    </div>

    <style>
      .grid-container {
        display: grid;
        height: 200px; /* Example height */
        width: 200px; /* Example width */
        align-items: center;
        justify-items: center;
      }
    </style>
    <!-- END COPY / PASTE -->