I would change the html instead of the css, this is easier.
Look at the shadowbox.js file an look for the "skin" object. Than change this as you like it. I hope this would work. Than you have to write something like this into the title-attribute title="Hallo<div class='small'>Hallo</div>"
I haven't test it yet and I don't know if its really work.
But just try it!