How limit the width of a fixed menu-bar on scroll?

You have another div controlling the full width background. You need to alter the css of that one if you want the width to look. This css takes the background off the full width div you have and put the background on the wrap div that you have maxing at 1200px;

.lifestyle-pro-blue .nav-secondary {background:none;}
.wrap{background-color:rgba(0,0,0,0.5);}