JSFiddle - React, Tailwind, and code Playground

HTML

<div class="content">       
    <div class="tlcontent"> 
        <div class="sideon" id="sideline0"> 
            somethings
        </div>
        
        <div class="trackon" id="trackline0">
            somethings
        </div>
        
        <div class="sideon" id="sideline1"> 
            somethings
        </div>
        
        <div class="sideon" id="sideline2">
            somethings
        </div>
    </div>
</div>

CSS

#populate {
    border: 1px solid red;
}

JavaScript

$(function() {
    var latest = $('.tlcontent div:last');
    latest.css({
        color: 'red'
    });
});