Brine gallery thumbnails toggle on/off

Hi! I’m using the Brine template (Wav) and trying to customize my the gallery on my page so you can click a button to view thumbnails. I have the button, but I’m stuck on how to show/hide the thumbnail strip. So far, I have added this to the code injection:

<script>function myFunction() {   var x = document.getElementById("the div");   if (x.style.display === "none") {
   x.style.display = "block";   } else {
   x.style.display = "none";   } }</script>

and this is the button on the page:

<button onclick="myFunction()">Thumbnails</button>

<div id="the div"><p> <br>
 Thumbnails here
</p></div> 

This is the link to the page: https://adrian-merrick-aks4.squarespace.com/becky-copy

Any advice would be greatly appreciated!!