#container {
display: flex;
position: absolute; /*fixed*/
width: 100%;
height: 100%;
left: 0;
top: 0;
background: #D2D2D2;
}
#container > div {
margin: auto;
width: 50%;
height: 50%;
background: tomato;
}
<div id="container">
<div>my content</div>
</div>