To do this, Open any text editor that is able to save a .css file (basically a pure text file), and paste the below code.
:link:active, :visited:active { outline: 3px auto; color: lightblue; }
Save the file somewhere, and name it something ending in .css. Then go into your Safari Preferences and click on the Advanced icon. From there, you can set a custom CSS file for Safari to use; point it to the file you just created. Of course you can take out the color: lightblue;, and Safari will use the same color of the link on each site. Or you can even change it to the color of your choice. To see the effects, you'll have to open a new web page; it won't change any already loaded pages.
This hint can also be used in Net News Wire and any other app that is able to use custom CSS files w/ webcore.

