OK – try this:
Go to the forum admin > themes > custom CSS
and paste in the following:
#spMainContainer .spTopicPostSection.spType-Admin .spUserSection .spProfilePopupLink { color: red; }
#spMainContainer .spTopicPostSection.spUsergroup-Sponsors .spUserSection .spProfilePopupLink { color: blue; }
That is aimed to work on the topics (posts) on the left where the user name is. Let;s see if it does.
Beware – there are spaces between some of the elements but not all. Best copy and paste it (use the ‘raw’ button)