Replacing Menu Links with Images

Hi!
I really wanted to use images as navigation links for my Verbosa theme. However, the custom CSS which I have also previously used in another (tumblr) Theme did not work:

nav li:nth-child(1) a {
display:block;
width:150px;
height:79px;
background-image: url();
background-repeat: no-repeat;
text-indent: -9999px;
}

How can I manage to change it for the Verbosa theme? It would be great if this could work out!
Thanks in advance
kimeyeleen