Your website and your site's content is yours, assuming that you owned the content to begin with. Site by Nite is just helping you get your content online.
As for the actual design, that belongs to Site by Nite and/or those who provide Site by Nite with designs. Should you decide that you want to move your site somewhere else, you can of course take all of your content, but you are not allowed to keep the design. This isn't really that big of a deal as you will probably be moving on to a bigger, custom designed site, so you really won't be needing the design anyway.