How can I add a 5px border on the bottom of the navigation menu in color #fcce11?  

That's not quite what I'm asking. I'd like to know how to add a border that runs full screen (left to right) on the bottom of the header.

I figured it out: 

#header-outer[data-full-width="true"] header > .container, #header-secondary-outer[data-full-width="true"] > .container {
    border-style: solid;
    border-bottom: thick solid #Fee011;
}