Thanks for writing in! It seems you are using default template which contains container and you have set max-width for the container from Customizer. So you need to set a No Container Template to get edge-to-edge full width layout – http://prntscr.com/amnbr4
So just to be clear, there are two places I need to select a template: The regular page editor and also the Cornerstone editor? This is confusing. Can you explain how this works and if I am understanding correctly please?