JSFiddle - React, Tailwind, and code Playground

HTML

<div id="theContainer">
    <div id="item">Content 1</div>
    <div id="item">Content 2</div>
</div>

CSS

#theContainer {
    height: 100px;
    width: 500px;
    position: relative;
    border: 1px solid #900;
    padding: 10px;
}
#item {
    height: 50px;
    width: 100%;
}

JavaScript

document.getElementById('item').style.borderTop = '1px solid #0ea2c7';