By enclose i meant put it inside a div. Just replace “YOUR BANNER” with the shortcode you were using and then add the css. That is it, sorry if i confused you by using the wrong terminology.
On a side note, see Tom’s advice on using wp_head hook for other things than scripts.
Please don’t hesitate to ask if you would like more clarifications.
Agressively support the kind of work you want to see. Buy it. Talk about it. Review it.