See official documentation here:
http://codex.wordpress.org/Function_Reference/wp_list_pages

This here is the most interesting part (towards the bottom):

They can be styled with CSS selectors:

.pagenav { … } /* the outermost list item; contains whole list */
.page-item-2 { … } /* item for Page ID 2 */
.page_item { … } /* any Page item */
.current_page_item { … } /* the current Page */
.current_page_parent { … } /* parent of the current Page */
.current_page_ancestor { … } /* any ancestor of the current Page */

In order to achieve an accordion menu effect for instance, the following CSS can be used:

.pagenav  ul ul,
.pagenav .current_page_item ul ul,
.pagenav .current_page_ancestor ul ul,
.pagenav .current_page_ancestor .current_page_item ul ul,
.pagenav .current_page_ancestor .current_page_ancestor ul ul {
display: none;
}

.pagenav .current_page_item ul,
.pagenav .current_page_ancestor ul,
.pagenav .current_page_ancestor .current_page_item ul,
.pagenav .current_page_ancestor .current_page_ancestor ul,
.pagenav .current_page_ancestor .current_page_ancestor .current_page_item ul,
.pagenav .current_page_ancestor .current_page_ancestor .current_page_ancestor ul { 
display: block;
}