In your custom CSS you can add
#nav ul li {
font-family: Tahoma;
}
to change the font family for the links. Tahoma can be changed to a different font for the desired effect. As an FYI, all computers may not have access to the same fonts that you do so if you apply a font on your computer that somebody else does not have installed only you will see it.
You can make changes to the background by doing something like
#nav {
background: red;
}
to make the background red.
#nav {
border-top: double 3px red;
}
would change the top border. Lots of options to change the look.
In regards to replacing links with images, it is possible though requires some work on your part.
You would have to find the dimension of the link that you want to create an image for. Then create the custom image for that link. Upload the image to your site and then in your CSS you would do something like
#nav ul li.menu-item-390 {
background: url('http://0.gravatar.com/avatar/e00501bf782b42d5db19ff75fca14f6a?s=128&d=mm&r=G') no-repeat top center;
}
to replace the Contact Me link with an image background. Note that for each link the above 390 would change depending upon the class of that link.
Contact Me = 390
About = 389
Home = 388