The CSS module adds, for users with enough permissions and enabled nodes, a CSS field on the node creation page.
Users might insert CSS rules in the CSS node field and those rules will be parsed on the node viewing.
This way CSS experienced users might create complex CSS based design for nodes contents.
IMPORTANT: note that the CSS editing permissions should be given only to trusted users (administrators). Malicious users which have this permission might broke your site design and also introduce security issues (XSS).
#D7CX: I pledge that CSS Module will have a full Drupal 7 release on the day that Drupal 7 is released.
See issue for more details and helping out!