Remove header / footer from single page

I need to have gallery pages with no header or footer (i.e. no branding or links) and I'm using Squarespace 7.1

I found this code at 38visuals, and inserted it into the page see attached jpegs. It does not work no matter what I try.

Anyone know what's wrong with the code or if this is even possible in 7.1

SS 7.1 use different header, footer tag.

Add this to page SEttings > Advanced > Header

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