As Sharp wrote in his post:

As such, I’ve made the raw icon files available here (at least until they revive the page). This will replace the blue (non-standard, sad looking) RSS icons from Safari with the one from the picture above.[Source]

The post also contains and talks about the following:

1) A link from where to download the RSS icons.

2) Instructions on how to upload and install the RSS icons.

A very useful chunk of information or knowledge for the Mac people.

Post from: EveryJoe

Changing RSS icons for Safari 3.1