Initiate the mobile header at your desired width. Change 768px to whatever you like.

gistfile1.txt

@media( max-width: 768px ) {
	.site-header,
	#site-navigation,
	#sticky-navigation {
		display: none !important;
		opacity: 0;
	}

	#mobile-header {
		display: block !important;
		opacity: 1;
		width: 100% !important;
	}

        #mobile-header .main-nav > ul {
            display: none;
        }

	#mobile-header.toggled .main-nav > ul,
        #mobile-header .menu-toggle,
        #mobile-header .mobile-bar-items {
            display: block;
        }
}