Adding a box around a menu item

Hi all! I was hoping someone could give me a hand.
I want to make the menu item “BOOK” more visible, by adding a box around the item, or even creating a clean white box with black letters so it stands out from the other menu items.

Would anyone know how can I do this? My CSS knowledge is zero, so I’d need a step by step guidance :).

Here the website:

Thanks a lot!

Try to add this CSS in Dashboard > Appearance > Customize > Additional CSS :

li#menu-item-316 {
border: 1px solid;
padding: 0 10px;

and you should get this:

DONE! Thanks so much. Works nicely.