When a customer initially makes a style change they will be given a little checkbox to revert back to the previous style if they don't like the one they chose.
If the customer tries a few styles and can't get back to the one they want, if the Sitebuilder 1.0 site has not been published with the new style, you can view the page source of the published site to find the Style in Sitebuilder 1.0, used and then search for it using the search box under the Style tab.