Update fancybox.css

Along with many other web authors, I set `* {box-sizing: border-box;}` for my sites. The unfortunate side effect is that the Fancybox boxes are mis-sized, making the display rather awkward (`#fancybox-title` box too big and circle-x for closure misplaced). Adding `box-sizing: content-box;` to `#fancybox-wrap` and `#fancybox-content` undoes this side effect by shielding Fancybox from the site's overall box model.