That is the .current-menu-item
CSS class that WordPress adds to let people know which page they are on.
I think if you play around with this code in your child theme:
#header-menu > li.current-menu-item {
background: /* relevant items here */
}
you can get it to match the other menu items. You may also need to define a style for the hover
pseudoclass. I would suggest using Firebug (Firefox) or Developer Tools (Chrome) to work on this, because menus can be tricky. (On the plus side, if you master styling menus, everything else is a piece of cake.)
This should get you started. Let us know how it goes.