JSFiddle - React, Tailwind, and code Playground

HTML

<div class='lightbox'>
    <div class='tc-content'>AAA</div>
</div>

CSS

.lightbox { 
    position: fixed;
    top: 0px;
    left: 0px;
    width: 100%;
    height: 100%;
    background: lightblue;
    text-align: center;
}

.tc-content {
    position: relative;
    top: calc(50% - 1em);
}