<li class="level<tmpl_var page.relDepth><tmpl_if page.isCurrent> current<tmpl_else><tmpl_if page.isAncestor> ancestor</tmpl_if></tmpl_if>">

.ancestor for styling the ancestors of the current page in a nav. Styling the parent is also possible, using the level* class or combining selectors like:

.menu li li.ancestor

...for the parent of a level 3 page