How do I adjust content width with Custom CSS

Hello,
After I recently updated Highend theme my layout was altered. The left padding or margin for the text boxes has been reduced. My content width in Highend theme options is set at 1140px with a boxed layout. Here is a link to one of the pages affected: https://www.magazine-advertisements.com/about-us/
Would I fix this with Custom CSS? If so, what would be the Custom CSS and where do I insert it?
Also, I have attached 2 screenshots of the page.
Thank-you.