I had a look at the source code for the index page, and this is what it says for me one the whos online bit.
Code:
<a href="member.php?u=18"><span style="font-weight: bold; color: blue;"><font size="2">[Daisuke]</span></font></a>
And as such, I couldn't see any css for it, I believe I have to edit the function that $activeusers calls, but am unfamiliar with the code used in that function, and don't know how to change it :/