JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="nav">
    <li>Home</li>
    <li>Content</li>
    <li>Help</li>
</ul>
<p>Original nav color is blue with a border-bottom of white</p>

CSS

ul {
    background-color:blue;
    height:50px;
}
li {
    color:white;
    list-style-type:none;
    display:inline-block;
    padding:7px;
    cursor:pointer;
}
li:first-child {
    border-bottom:2px solid white;
}
li:hover {
    border-bottom:2px solid white;
}

JavaScript

var nav = document.getElementById('nav');

if (nav.style.backgroundColor == '') {
    nav.style.backgroundColor = 'green';
    nav.firstChild.nextSibling.style.borderBottom = '2px solid yellow';
}