If you go into style.css (another option other than using the custom CSS feature)
around line 828, in the Menu section of the code, you should see this code
#header-right .widget ul.menu li:hover > a,
#header-right .widget ul.menu a:focus,
#header-right .widget ul.menu .current-menu-item > a,
#header-right .widget ul.menu .current-menu-ancestor > a,
#header-right .widget ul.menu .current_page_item > a,
#header-right .widget ul.menu .current_page_ancestor > a {
color: #7c9b30;
}
In the color: section at the bottom, put in your desired hex color