How to change default username color?

We recently updated to a new theme which is fairly light colors,and the default username color was white..so I made a members group for people to automatically join, color=black. But th people that were I groups like the girls group who were red,are now black.only mods and admins have different from black,so how do u change the default username to black so that If I put them in a group it changes there color?

Thank you very much! All I did was out members at bottom,thank you