Highlight parent when sub page is not part of the menu

Hello there,

I currently have a page that is not part of my menu system. The page has a parent page, in that it’s a sub of a page that’s in the menu. I would like to have the parent page item highlighted in the menu when the sub page is being viewed.

I am currently using the following code for the current page highlight:

#menu-main-menu li.menu-item-21.current_page_item a {color:#ed1b34!important;}

Can anyone tell me how to achieve highlighting the parent item when the sub (excluded from the menu) is being viewed?

Thanks in advance