Kind of a strange question. HTML is HTML, and you can put the inner HTML of a Marketo LP inside the <html></html> of any other page.
Of course if any relatively-referenced CSS, JS, and images don't exist on the other site, those are all going to break. And you're going to inherit the CSS and code of the other site, which most likely will wreak havoc on the newly pasted code -- unless the other site had no scripts or styles at all to begin with.
There's no way to magically avoid these results, you can only fix the conflicts afterward.
Now, the other thing you can do is load the IFRAME from your non-branded, but secure, LP domain: the link will look like https://na-ab13.marketo.com/lp/AAA-BBB-CCC/unsubscribepage.html. Substitute your Munchkin ID for "AAA-BBB-CCC" and your pod id for the "na-ab13" (for example, if your instance is on app-sj03 use "na-sj03"). However, while going back to an IFRAME remedies all the styling concerns, if you do this you will need to add additional JS on the outer page and inner page in order to restore tracking, since the cookie will not automatically be shared between .snowboundsoftware.com and .marketo.com.