SOURCE

<div class="outside">
    <div class="inside"></div>
</div>

div.outside {
    background-color: skyblue;
    width: 350px;
    height: 350px;
    
    display: flex;
    justify-content: center;
    align-items: center;
}

div.inside {
    background-color: lightgreen;
    width: 100px;
    height: 100px;
}