Try this, "Appearance > Theme Editor" select theme.css file that you're using.
Search for:
.layout_core_menu_main .navigation > li > a:hover
Add this above:
.layout_core_menu_main .navigation > li.active a:link,
.layout_core_menu_main .navigation > li.active a:visited
{
color: #e376bf;
background: #9ea9b0;
}
Change colors as needed. Don't forget to clear cache when testing.