If you don't like Apple's style for the Safari Reader feature, you can edit the stylesheet in Reader.html. In Safari 5.0, the file is at:

/Applications/Safari.app/Contents/Resources/Reader.html

In Safari 5.1 the file is at:

/System/Library/PrivateFrameworks/Safari.framework/Versions/A/Resources/Reader.html

Detailed instructions are available for making the changes in Safari 5.0 and for Safari 5.1.