How to deal with iframe with DOMContent when user clicked something?

I have an website with content, with div class card2 which has white background and some buttons (button1, button2, button3)

I have dashboard, where I load an website content using iframe (full html)

So, when user opens his dashboard, I apply the code bellow which will remove a white background from style and buttons will be on the background without white color

<script>

  window.addEventListener('DOMContentLoaded', (event) => {
    console.log('DOM fully loaded and parsed');
    document.getElementById("biolink_preview_iframe").load = function(){}
    let iframeName = document.getElementById("biolink_preview_iframe");
    let iframeContent = iframeName.contentDocument;
    iframeContent.body.innerHTML = iframeContent.body.innerHTML + "<style>.card2{background-color: unset !important;box-shadow: none !important}</style>";

  });

</script>

When user change the order of a buttons in dashboard, it reloads and iframe, and my code above is not working anymore, white background is visible again.

Can you suggest me please, why this happening?

I tried this code

<script>

  window.addEventListener('DOMContentLoaded', (event) => {
    console.log('DOM fully loaded and parsed');
    document.getElementById("biolink_preview_iframe").load = function(){}
    let iframeName = document.getElementById("biolink_preview_iframe");
    let iframeContent = iframeName.contentDocument;
    iframeContent.body.innerHTML = iframeContent.body.innerHTML + "<style>.card2{background-color: unset !important;box-shadow: none !important}</style>";

  });

</script>

>Solution :

Typos.

You need either

document.getElementById("biolink_preview_iframe").onload = function

OR

document.getElementById("biolink_preview_iframe").addEventListener('load', function

and you have a {} after function. You likely meant

window.addEventListener('DOMContentLoaded', (event) => {
  console.log('DOM fully loaded and parsed');
  document.getElementById("biolink_preview_iframe").addEventListener('load', function() {
    let iframeContent = this.contentDocument;
    iframeContent.body.innerHTML += "<style>.card2{background-color: unset !important;box-shadow: none !important}</style>";
  });
});