I'd like to learn more about Safari Reader, too - especially the paginated-article collation feature, which is a real advantage over Readability.

Anyway, as for tweaking the stylesheets - here are some links to compare.

Here is the original Readability stylesheet: http://lab.arc90.com/experiments/readability/css/readability...

And here is my version (featuring an inexpert mix of px, em, and % units that surely could be improved): http://anoved.net/css/mcreadability.css

To try your own modifications, edit a Readability bookmarklet and set _readability_css.href to the URL for your stylesheet.