Move primary menu below header, make header area full width

I want to move the menu beneath the header. Also, but less important, I’d like to make the entire layout ‘less-boxed’: give the header div a full-width black background and perhaps smaller side margins on all pages. I’ve established a child theme which is presently empty. I tried the WP Sticky Menu plugin which is helpful, but I’d prefer to simply have the menu beneath the header.

I tried tips given at Make a Full Width Twenty Sixteen Theme Header but that didn’t work right off. I previously spent much time trying to do this in 3 other themes (including 2017) without success, so I thought I should ask the experts before diving in again.

Thanks in advance for any suggestions, even if it’s another responsive theme.

The page I need help with: [log in to see the link]