Can not get any form of Modal to work

Hello!
I tried a lot of ways to get a popup or modal to work, with and without js, but everytime I click on the link that is supposed to open the modal, the page only scrolls up and nothing happens.

This is one of the alternatives I was trying out:

<style type="text/css">#overlay {
     visibility: hidden;
     position: absolute;
     left: 0px;
     top: 0px;
     width:100%;
     height:100%;
     text-align:center;
     z-index: 1000;
     background-color:rgba(0,0,0,0.8)
}
#overlay div {
     width:300px;
     margin: 100px auto;
     background-color: #fff;
     border:1px solid #000;
     padding:15px;
     text-align:center;
}</style>
<script type="text/javascript">function overlay() {
	el = document.getElementById("overlay");
	el.style.visibility = (el.style.visibility == "visible") ? "hidden" : "visible";
}</script>

<a href='#' onclick='overlay()'>Click here to show the overlay</a>

<div id="overlay">
     <div>
          <p>Content you want the user to see goes here.</p>
     </div>
</div>

Since I tried various methods, for example with pure css and ::target, I figured that it’s maybe not the code, but something I am doing wrong so the code won’t work well with the theme.Do you know why the modals don’t work? I do not want to use any form of plugin to make a modal, because that really should not be necessary for this normally easy task.
Any hint appreciated. Thank you very much!

Kind regards