There might be copyright infringement problems when you copy straight off like with the clone themes. When it comes to taking css/html here and there I don't really think its a problem. No piece of css/html is so totally unique as to warrant copyright status. The whole page might have it but not 3,5,8 lines of HTML or CSS. Heck its even possibly that none of the code on the page is unique only the images. Its similar to the whole code patent crap. Amazon has a patent for using a cart on their homepage I think.