@Lisa J What do you mean by "wrap it"? The outer wrapper for the embedded form is just an HTML FORM tag. You can style it any way you like.
As @Mike points out you can have custom CSS come in along with the form itself, though if the CSS selectors refer to elements that aren't inside the FORM tag -- but, rather, wrapper els in your external page -- that could be confusing to understand. (It will still work, of course, since the CSS will be in page scope, but you could be left thinking, "Why does the form's custom CSS in Marketo refer to some random DIV?")