I took a look at the theme documentation and there doesn’t appear to be container width controls available in the backend.
The proper way then to adjust this is to create a child theme from the main theme, and change the theme styles via css.
Without seeing the theme files I couldn’t tell you specifically where that is, other than the width of the content is determined by the class .container-fluid but more importantly, when I adjusted the width to 800px in the inspector, I found the site breaks considerably.
That means there’s a considerable amount of css adjustments that need to be made in order to make an 800px width viable. All of its media queries are based on the size of the browser window (as it should be), not the content container. You’ll need to do a lot of work to make this theme function at that width on desktop.