how do I change fluid width theme to fixed width?!

Please help, I’ve been trying this for days.
I’m trying to change my WordPress site’s theme from a variable width to a fixed width. Apparently this is easy to do, it was suggested on the theme’s launch site that it would just be something simple such as this:

For everyone who wants to make it a fixed width:
Just create a div called, say “boxer”, and add this to style.css:

#boxer {
margin: 0 auto;
width: 960px;
}

Then insert the #boxer div into the header.php before the #header div. Then end the #boxer div at the very bottom of the footer.php.

//

However, I just tried to do this (though perhaps I wasn’t doing it right, I really need to know exactly what to insert and where) and it didn’t seem to do anything.
What’s weirder is that I am pretty sure I did this successfully about a year ago and changed it back for some reason, but I can’t remember how I did it.

My site is:
https://cityrat.com
(test site (same theme) is:
https://furgarden.com)

and the theme i’m using is evidens white from design disease.
https://designdisease.com/blog/evidens-wordpress-theme-release/
The people who made the theme aren’t very helpful because it’s a free theme i assume.

I would be most grateful for any assistance you can provide.

thanknssssssss!!!