Custom navigation CSS

[steveshead]

I'm trying to replace the navigation CSS with my own. In the past I've used plugins with the following code in the functions.php.

// Reposition the Primary Navigation
remove_action('genesis_after_header', 'genesis_do_nav');
//add_action('genesis_before_header', 'genesis_do_nav');
add_action('genesis_after_header', 'custom_do_nav' );
function custom_do_nav() {
    cmm_display_menu(ID);
}

My question is how do I apply my CSS to the navigation? I believe I'm thinking to hard about this, I just can't seem to think around it this time. Help!