Seeking way to adjust max-width for 1200px+ on a Brine (Mentor) site

I'm having a strange problem with my site where it appears that the max-width container on screens above 1200 px goes down to 33%. It looks lousy and I'd like to change it, but I can't figure out how to do so. 

The site is https://www.ofbyforall.org/ and the problem occurs on blog posts like this one.

Using the inspector, I was able to figure out that the width of the container goes from being 100% for every screen smaller than 600px 83.33% for screens 600px - 900px 66.67% for screens 901px - 1200px Then at 1201px, something happens and there's no media query at all, and it defaults to the rule that the width should be 33%. See attached image. This makes blog posts and pages look lousy unless I manually insert spacers on the sides, which I don't want to do. 

Is there a way I can change this 33% default to something more like 66%? Or is there a way to add a media query to the CSS on the site for screens up to 2000px or something? I would greatly appreciate any CSS suggestions people have for how to solve this.