Unfortunately, it is not that simple. To be able to alter any of this, you need to publish your site to a folder so that the html files are created. iWeb does not create these files until published. You then need an html editor to be able to change any of the code or CSS, so you would need to import the site into something like Text Edit or Text Wrangler and then change elements of coding and CSS.
Unless you really know what you are doing with hand coded html, then it really is not worth it. You would do better to just re-arrange your site a little, turn off the original nav bar and then create your own. If you do this, you will also find that you site is located by Google more effectively too.
I have to say that I create my own nav bars for my site, as mostly, I don't like the ones provided by iWeb templates.