I’ve always thought it was a neat idea to be able to apply custom CSS stylings to different posts on a blog, allowing you to make subtle tweaks to the overall design on a case-by-case basis. A post about an upcoming movie could have a large poster as the page’s background image, the colors could be changed to match, etc..

Digging into WordPress has a post with two methods that achieve just that effect. One adds a meta box to your Write Post screen, where you can input CSS that will be echoed out on single post pages. The other allows you to drop a numerically-named CSS file into your theme folder, which is called upon when appropriate.

Custom CSS Per Post [Digging into WordPress]