in Custom CSS:

body .wrapper header #nav ul > li:first-child {
backgound-color: green;
background: url('http://....link-to-file');
}
body .wrapper header #nav ul > li:first-child a {
opacity:0;
}

in Custom JS:

jQuery(document).ready(function(){
jQuery('body .wrapper header #nav ul > li:first-child').click(function(e){
window.location.href = "http://cosmothemes.com";
event.preventDefault(e);
});
});