Menu function

Hello every1 ??

The menu i will talk about is the one under images at https://www.comunesampe.altervista.org/
I’ve created this menu by myself and i’d like to have it automatic with items in the WordPress Menu called “Menù“

The default usage is now:

<ul id="topnav">
    <li><a href="URL">TITLE</a></li> <strong>(Single Item)</strong>
    <li>
        <a href="URL">TITLE</a> <strong>(Item with sub-items)</strong>
        <span>
            <a href="URL">TITLE</a>
            <a href="URL">TITLE</a>
            <a href="URL">TITLE</a>
        </span>
    </li>
    <li><a href="URL">TITLE</a></li> <strong>(Single Item)</strong>
</ul>

Here the css used:

ul#topnav {
	margin: 0; padding: 0;
	float: left;
	width: 100%;
	list-style: none;
	position: relative;
	font-size: 16px;
	background: url(https://newwpthemes.com/livedemo/wp-content/themes/goBusiness/images/nav-hover.png) repeat-x;
	font-family: Tahoma;
}
ul#topnav li {
	float: left;
	margin: 0; padding: 0;
}
ul#topnav li a {
	padding: 10px 15px;
	display: block;
	color: #ffffff;
	font-style:uppercase;
	font-weight: bold;
	text-decoration: none;
}

ul#topnav li:hover {
	color: #000000;
}

ul#topnav li span {
	float: left;
	padding: 5px 0;
	position: absolute;
	left: 0; top:30px;
	display: none;
	width: 100%;
	font-size: 12px;
	background: #ffffff;
	color: #fff;
	border: solid #e0e0e0 2px;
	z-index:999999999999999;
}
ul#topnav li:hover span { display: block; }
ul#topnav li span a {
	color: #000066;
	display: inline;
}
ul#topnav li span a:hover {text-decoration: underline;}