Ladies and Gentlemen,
I just realized, that in the /resources/openacs-default-theme/styles/default-master.css file, visited links get the same color as unvisited links:
This is number 3 among the top 10 mistakes in web design
by Jakob Nielsen. Therefore, I would recommend to simply remove this a:visited statement for visited links for the default theme. Alternatively - if you want to have the same color on all browsers - you could use the value #551A8B, which Google seems to use on its result pages.