Snippets: Center Mobile Menu

/* Positon mobile menu under logo
/* Will no longer work with fixed header without additional tweaks */
body #mobile-menu {
    clear: both;
    position: relative;
    right: auto;
    top: auto;
    margin: 0;
    padding: 20px 0;
}

/* Make mobile menu span full-width and tweak background */
body #mobile-menu .mobile-menu-toggle {
    width: 100%;
    float: none;
    border: none;
    background: #000;
    color: #fff;
}

Result:

All CSS snippets should be added via a child theme's style.css file or using the built-in Custom CSS panel.