Place the file in your user account’s Safari folder: ~/Library/Safari/nounderlines.css
In Safari..Preferences.. and in the Advanced tab..Stylesheet - choose the nounderlines.css file.
Sadly, the hard drive in my beautiful G5 has melted and until I can replace it I'm stuck using Windoze on my work laptop.
The upside is that I tried this fix for underlined links, and it works magnificently.
I created the original document as a text file using WordPad, then saved it in C:\Program Files\Safari.
Preferences are found in the "Edit" menu in Windows XP, and you can simply select "Other ..." in the "Style Sheet" pulldown menu and direct the browser to the nounderlines.css file.