A really quick example of switching between desktop and mobile page layouts at 800px:

  @media (min-width: 800px) {
     #page {
        grid-template-areas:
           "header header"
           "menu   content"
           "footer footer";
     }
  }
  @media (max-width: 800px) {
     #page {
        grid-template-areas:
           "header  menu"
           "content content"
           "footer  footer";
     }
  }