JSFiddle - React, Tailwind, and code Playground

HTML

<div id="myGrid1_headDiv" class="gt-head-div">
<div class="gt-head-wrap">
<table id="myGrid1_headTable" class="gt-head-table">
<tbody>
<tr class="gt-hd-row">
<td class="gt-col-mygrid1-uid">
<div class="gt-inner gt-inner-left" unselectable="on" title="#">
<span>#</span>
<div class="gt-hd-tool"><span class="gt-hd-icon"></span>
<span class="gt-hd-button"></span>
<span class="gt-hd-split" style="cursor: col-resize; "></span></div></div></td>
<td class="gt-col-mygrid1-p_deldate">
<div class="gt-inner gt-inner-left" unselectable="on" title="Planned Delivery Date">
<span>Planned Delivery Date</span>
<div class="gt-hd-tool"><span class="gt-hd-icon"></span>
<span class="gt-hd-button"></span>
<span class="gt-hd-split" style="cursor: col-resize; "></span></div></div></td>

JavaScript

$("div.gt-inner:first-child span").addClass("celltitle");