Full width changed to fixed width without my knowing how.

I downloaded the dummy content to begin working on my site and the template was full-width in the browser window, which is what I wanted, and then all I did was begin to edit the menu, and when I viewed the site in the browser, the full width was gone and the template is now fixed width with a gray background. Can you tell me how to change it back? I don’t know what I did to change it.
Thank you in advance