How do I get the site to center on a large screen?

You seem to be asking for 2 different things:

1. How do I get the site to center on a large screen?
2. how the entire site fits on the screen.

While it may seem like that, all I am really trying to do is have the entire site centered whenever the screen is larger than the default width. I just happen to be working on a wide screen.

This ?

.site {
    margin: 0 auto;
}

Where in the css doc does this go?

If you’re not using a Child Theme, and if the theme doesn’t have a designated section of the dashboard for CSS modifications then install this Custom CSS Manager plugin https://www.ads-software.com/plugins/custom-css-manager-plugin

Then use its “CSS Code” section of the dashboard to hold your CSS modifications.

Alternatively use your Child Theme style.css file to hold your CSS modifications.

to add that css, you can go to appearance / editor and edit style.css (should edit that by default).

alternatively it will be much easier any of a billion other themes, lol

to add that css, you can go to appearance / editor and edit style.css (should edit that by default).

That is not the recommended way to make modifications. Were you aware that theme updates erase all of those types of modifications? Also, if there’s a problem in future it’s a whole lot harder to debug when you can’t clearly see what you’ve changed in comparison to the original theme.