The CSS and layout templates for Safari 2 are located in /System: Library: PrivateFrameworks: SyndicationUI.framework: Versions: A: Resources: Themes: default.syndtheme: Contents: Resources; the CSS file itself is called Articles.css. By changing this file, as well as the other templates there, you can change the way Safari's RSS pages look.

Open a Terminal, and then sudo su root to get to the files, and then do this:

$ cd /System/Library/PrivateFrameworks/SyndicationUI.framework/Versions/
A/Resources/Themes/default.syndtheme/Contents/Resources/

You can pico them from there, or cp them to your /Users/ folder and tweak them there. If you edit any of the files in Photoshop, make sure you remember to chmod 777 the file before hand. Then after you edit the files, just use cp to copy it back to the original location. Enjoy the fun!

[robg adds: I haven't tested this one. Remember to make a backup of anything you're going to modify, and you should probably quit Safari before making any changes. You'll need to know something of CSS in order to change much of the page, but there are also some graphics files there that can be changed. Finally, I would personally use sudo cp (to move any of the files around) or sudo pico (to edit as root) without actually enabling a root shell, as I'm a bit paranoid about running root shells and doing something stupid.]