JSFiddle - React, Tailwind, and code Playground

HTML

<div class="transparent_background">
    <div id="window">hello.</div>
</div>

CSS

.transparent_background {
    position:fixed; 
    top:0px; left:0px; 
    width:100%; 
    min-height: 100%;
    background-color: rgb(0, 0, 0);
    background-color: rgba(0, 0, 0, 0.3);
    filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#99000000, endColorstr=#99000000);
    -ms-filter: "progid:DXImageTransform.Microsoft.gradient(startColorstr=#99000000, endColorstr=#99000000)";
    z-index:990;
}

#window {
    border:2px dotted red;
    background:white;
    
    position: absolute;
    top:0;
    bottom: 0;
    left: 0;
    right: 0;
    margin: auto;
}