Constraining header width in Burke/Brine template.

Hi. I've used CSS to force the navigation in my header to always the mobile version, but I'd also like to restrict the width to line up with my page content. Can anyone help me with that?

The site in question is:

http://mattpruett.com

Here's what I'm using for the forced mobile nav:

[data-nc-base="mobile-bar"] {
  display: block;
}
.Header {
  display: none!important;
}