<div class="container-grid style-parent">
    <div class="style-texte">
      I'm center with flexbox
    </div>
</div>

.container-grid {
  display: grid;
  place-content: center;
}

.style-parent {
  background: green;
  width: 500px;
  height: 250px;
  border-color: white;
  border-style: solid;
}

.style-texte {
  color: white;
  border- color: black;
  border-style: solid;
}