The thread titles (and the individual post #'s) that I've clicked on are shown on my screen in dark blue, while the ones I haven't clicked on are shown in medium-dark blue. These can be very difficult to distinguish. It would be a lot easier on the eyes if there were a greater color contrast. .....like blue and red, for instance.
If I understand you correctly, you can change the color of the links you have visited (while browsing with IE) by clicking on the Tools menu, then select Internet Options, then click on Colors (located at the bottom left of the dialog box), then modify the color assigned to links you have visited and to links that have not been visited.
Changing Link (& text & background) colors: