Code breaking site design! How do I fix?

Hi,

I recently added a basic code block to a page (not on footer) to remove the header and footer (we needed a 'landing page' to be reached by URL but hidden on the site, without showing the site nav or connecting to other pages). I found this code by Googling it, and at first, it worked. (source: https://www.mehdiaoussiad.com/blog/squarespace-remove-footer-and-header-from-one-page)

<style>
  .header, #footer-sections{
    display:none !important;
  }
</style>

Now, the design is breaking across our site! Whole chunks of text and buttons have gone missing, fonts have swapped, photos are blown out and wonky, etc. I am not an experienced developer, and need your help!

Can someone please:

1) Tell me how to do this correctly (if that's possible to do) and/or
2) Tell me how to remove this code block from our site (I can't find it anywhere now) and
3) Tell me if removing the code will make the site secure again

Many thanks!

1 hour ago, Connectary said:

Now, the design is breaking across our site! Whole chunks of text and buttons have gone missing, fonts have swapped, photos are blown out and wonky, etc. I am not an experienced developer, and need your help!

Can you share your website URL and the page URL where you think the code block is?

My feeling is that if you copied the code perfectly, it wouldn't cause these problems, and even if there was a major error, it would only affect this one page. Is there anything else you added to the general code injection?