There are 3 ways to edit a page with iWeb...
1. Using only iWeb.
2. Modifying the page after it is created.
3. Editing the template files.
Every one of this has its own advantages and disadvantages, for example, you cannot add Java or JavaScript or any other code using iWeb or changing the templates, however you can add it after the page is created but iWeb will clean it every time you update the page. Some style details like link colors, navigation bar and blog fonts/styles can only be changed altering the template file, although you can do this after the page is created, the only way to create permanent changes that won't be cleaned after updates is to change the template file. Give it a try before trying anything too radical, iWeb can do a lot by itself if you use your imagination.