I am garbage at this and know very little css or coding. What I would like to do is have my primary navigation menu bar stretch all the way across the page, but have the content in the pages (the white part or the page) be much smaller and possible have the grey border around it.
Looks like your Navigation bar is already stretch all the way across the page?
Make sure the Navigation Width and Inner Navigation Width in Customizer > Layout > Primary Navigation are both set to Full.
You can change the spacing between each menu item to stretch them out in Customizer > Layout > Primary Navigation > Primary Menu Items > Left/Right Spacing