CustomReader is a Safari 6 extension that lets you easily customize the look of Safari Reader.
You can change basic formatting options, such as font and color, in an easy-to-use settings panel. For greater control, the Advanced tab of the panel lets you edit a custom stylesheet to override any built-in CSS rule or property of Safari Reader.
In addition, you can have CustomReader automatically activate Safari Reader whenever you visit pages that you specify. (You do so by specifying regular expressions to match specific URLs.)