Menu centering

You need to update your SSL cert: “This server could not prove that it is francombat.com; its security certificate expired 571 days ago. ”

This CSS centers your menu:

nav.classic_menu {
    position: relative;
    float: none;
    text-align: center;
}

To add or override CSS: use the “Additional CSS” option in the customizer. https://codex.wordpress.org/CSS#Custom_CSS_in_WordPress

Learn to use the Chrome Developer Tools or Firefox Developer Tools to help you see and test changes to your CSS.