Changing width of one page on my website

Hi there, I’m trying to change the width of a single page and not the whole website. This is what I have currently and outlined in red is what I want it to look like for my photos. I am using the Horizon theme if that helps.

Go to the section settings of the section where you want to override width and add this into Custom CSS setting:

{
  --page-width: 800px; /* amend to your need */

  /* some sections use one of these -- default values below to override */
  --narrow-content-width: 36rem;
  --normal-content-width: 42rem;
  --wide-content-width: 46rem;
  --narrow-page-width: 90rem;
  --normal-page-width: 120rem;
  --wide-page-width: 150rem;
}

This code will only work in section “Custom CSS”.

Repeat for other sections on this template.

Thank you very much, I really appreciate it!