Can I change the font of the nav bar menu item only?

Hi,

everything is in the title.
By using firebug I can see I need to select this but should I use all these id to change only the font and what about the sub menu?

 .header_color .sub_menu > ul > li > a, .header_color .sub_menu > div > ul > li > a, .header_color .main_menu ul:first-child > li > a, #top .header_color .main_menu .menu ul .current_page_item > a, #top .header_color .main_menu .menu ul .current-menu-item > a, #top .header_color .sub_menu li ul a {
  color: #808080;
  font-family: 'Playfair Display';
}

And should I declare the font somewhere or because it is a google font and some seem to be already declared in the theme (like playfair display) I just can write font-family: ‘Playfair Display’ ?

Thanks a lot!

Chrys