Any idea how to achieve this by CSS or script in nodebb ACP?

link :https://www.tuhh.de/MathJax/test/sample-dynamic.html

you can easily check the source code of it, here I copy it in below

<!DOCTYPE html>
<html>
<head>
<title>MathJax Dynamic Math Test Page</title>
<!-- Copyright (c) 2010-2015 The MathJax Consortium -->
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<meta http-equiv="X-UA-Compatible" content="IE=edge" />
<script type="text/javascript" src="../MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>
<style>
input {margin-top: .7em}
.output {
border: 1px solid black;
padding: 1em;
width: auto;
position: absolute; top: 0; left: 2em;
min-width: 20em;
}
.box {position: relative}
</style>
</head>
<body>
<script>
//
// Use a closure to hide the local variables from the
// global namespace
//
(function () {
var QUEUE = MathJax.Hub.queue; // shorthand for the queue
var math = null, box = null; // the element jax for the math output, and the box it's in
//
// Hide and show the box (so it doesn't flicker as much)
//
var HIDEBOX = function () {box.style.visibility = "hidden"}
var SHOWBOX = function () {box.style.visibility = "visible"}
//
// Get the element jax when MathJax has produced it.
//
QUEUE.Push(function () {
math = MathJax.Hub.getAllJax("MathOutput")[0];
box = document.getElementById("box");
SHOWBOX(); // box is initially hidden so the braces don't show
});
//
// The onchange event handler that typesets the math entered
// by the user. Hide the box, then typeset, then show it again
// so we don't see a flash as the math is cleared and replaced.
//
window.UpdateMath = function (TeX) {
QUEUE.Push(HIDEBOX,["Text",math,"\\displaystyle{"+TeX+"}"],SHOWBOX);
}
})();
</script>
<p>
Type some \(\rm\TeX\) code and press RETURN:<br />
<input id="MathInput" size="80" onchange="UpdateMath(this.value)" />
</p>
<p>You typed:</p>
<div class="box" id="box" style="visibility:hidden">
<div id="MathOutput" class="output">$${}$$</div>
</div>
<script>
//
// IE doesn't fire onchange events for RETURN, so
// use onkeypress to do a blur (and refocus) to
// force the onchange to occur
//
if (MathJax.Hub.Browser.isMSIE) {
MathInput.onkeypress = function () {
if (window.event && window.event.keyCode === 13) {this.blur(); this.focus()}
}
}
</script>
</body>
</html>