Keep the main menu button highlighted when at sub-pages?

I wonder how I’ll keep the main menu button highlighted while my sub-pages are chosen? I have already made the main page as parent of the sub-pages in admin panel. I use fruitful theme.

And I’ve tried this without success:

.menu .current_page_item a, .menu .current_page_item a:visited, .menu .current_page_parent a {
background-color:#9e8ba0;
}

Thanks for any input!

Hanna