My site is
The line appears below the header and above the first post and sidebar.
I found that if I add a menu, the line goes away in favour of a menu bar. But in this case, for the moment anyway, I don’t want a menu.
Followup to that, is there a way to remove the right sidebar from blog posts? I’d like to have them full width. I see a page template for that, but nothing for the blog posts.