Add an id on post which is in a category

Hi, I'm creating a website based on your theme but I have a problem. I need for my css rules to have the id of the current category put in the body when I'm in the article. For examples : I'm published an article in the category "Medias" so when i am in the "Medias" category, i've the id "category-medias" wich appears on the body. But when i clicked on the article, the body id "category-medias" desapear and is deplaced on the post div which is in "content". But i need the id body "category-medias". Sorry for my english and thanks for your help!