How to Center the Navigation Menu

Add the CSS code below. How to Add CSS.

@media (max-width:3000px) and (min-width:881px) {
/* Main Nav */
ul.topnav { text-align: center;}
ul.topnav li {float: none; display: inline-block; }
ul.topnav li ul li {float: left; display: inline-block; }
/* For WPcart Menu Plugin */ #wpmenucartli { display: inline-block; float: none !important; }
}