Single post in Site Skins: How-To, and Helpful CSS

Hi all! I'm really sorry if I'm missing something obvious here, since I'm a complete noob at this sort of stuff, but there were a couple of skins I was hoping to try out and none of them seem to work... I copy all the code and paste it into the skin path box, but everything always remains as default. Do I need to modify the code in some way for it to work? Thank you!