1) I misunderstood your question, I thought you where asking about the Wizard, not the CSS file directly.
the best way to determine which style controls a specific element is to use a tool like Fire Bug in Fire Fox. Fire Bug allows you to select elements on the page, and see the CSS Styles that are controlling that element. it also allows you to make changes to the css and see the results in the browser for experimenting.
see the following road map article on using Fire Bug:
roadmap_10.php
2) The wizard does not allow you to set individual background images for the main level items, you would need to edit the menu code and a an inline background style to accomplish that.
For example, the top level menu item may be:
<li><a href="home.html">Home</a></li>
to give that item a unique background , add an inline style to the anchor:
<li><a href="home.html" style="background-image:url('plugins/images/home.jpg');">Home</a></li>
