Fixed width container for Antimatter child

I've tried following the instructions here to create a centred, maximum width container on my site. It works when I disable slidebars.min.css but not otherwise, and of course disabling slidebars messes with the presentation in the footer, for some reason.

I can't seem to determine a more specific selector. Would it be best to add a new selector in the template, or is there some better approach?

Thanks

You can use the following rules to change the default desktop browser display of Antimatter. You'll probably need to mess around with the explicit widths, but this will get you most of the way.

These can be added to either your _custom.scss or custom.css.

// Over-ride default Antimatter behavior of full-browser-width
#sb-site,
.sb-site-container,
.sb-slidebar,
body,
html {
  max-width: 1200px;
  margin: 0 auto !important;
}

#header #navbar {
  max-width: 1000px;
}

.scrolled#header { 
  left:0;
}