Change Width of Gallery Section Masonry (7.1)

Site URL: https://www.rachelwongdesigns.com/illustration

Hi, I'd like to make the width of the gallery section of my illustrations smaller (like the width of M, in the usual Squarespace content blocks). I can't figure out how to change this width.

The site password is: theotheo

Thank you in advance!

I see white blank page here. Can you add gallery?

@tuanphanHi tuanphan, it's showing on my end? I'm not sure what to do:

Add to Design > Custom CSS > Then save & reload the site

/* Illustration */
body#collection-61d60e85d54fab22b3ed93ce .gallery-masonry {
    max-width: 900px !important;
}