Please be sure to read this article before starting your editing > http://csswiz.wordpress.com/2009/10/15/if-you-have-the-wp-com-css-upgrade/
Also note that CSS is theme specific here at wordpress.com, there is no official Staff support for the CSS upgrade, and precious little Volunteer support. Please be patient while waiting for one of the two Volunteers who help to sign in and locate this thread.
You can click this link and see if the question has been asked and answered previously. http://en.forums.wordpress.com/tags/pilcrow
Also note that you need to allow for the navigation bar at the very top of the blog which I believe is 22 pix in height.
Browser’s developer tools for CSS editing > for Firefox, install Firebug. For Internet Explorer, install the Developer Toolbar.