I done what WPyogi said using jetpack
You should not be making changes to the theme files – they will be overwritten and erased when WP is updated. Move your existing CSS changes to a child theme or custom CSS.
The add this to the new CSS:
#access .menu-header li#menu-item-639 {
background-color: red;
}
If you just want the font color to change use “color” rather than “background-color” – and obviously, use whatever color you want.