joomla uses user3 for top menu display.  That is user3 module position and topmenu from menu manager.

It is always a nice display when the menu item is active to its related content.  For example, if you are in "contact us" page, the "contact us" menu should be active and be in different style.  So, the menu will look good.

For this first you need to set the "Full active highlighting" in the module manager for the "top" position. 

Secondly,  you should add the following css to the template_css.css file or anyother file if you are using separate css files,

ul#mainlevel-nav li a#active_menu-nav
{   
    /* any style */

}