fullwidth header wider than body

Hi, just testing out the theme and stumbled on my first little question: I want the background color for my header to stretch fullwidth -- it seems I have to set the header to fullwidth to achieve that -- but I don't want the header content to be wider than my body content, as it seems to be when I set the header to fullwidth (see https://www.screencast.com/t/26rhcfruk). Is there a setting I'm missing?

Thanks!