Set the main menu item to current when a subpage is active and style it. From CobaltApps forum http://cobaltapps.com/forum/forum/main-category/web-design-talk/85261-make-menu-item-active-when-sub-menu-item-is-chosen-help-please
li[class*="current"] > a {
background-color: #CC0000;
}/* nav current item highlight - children */
.genesis-nav-menu li.current-menu-ancestor a {
background: #98B33E;
}