I would most likely say the same thing you said, "I thought is was plain text (except for emoticans). I am probably just highlighting my ignorance.", if it wasn't for me being way to poor to make all the long distance phone calls required to communicate with the people I know. Nevertheless, as a result of my necessity to use messengers and such, I can truly tell you that YES you can (are supposed to be able to) make many variations between your fonts and almost the entire way that your messages look. You are supposed to be able to create your own personal IM stationary.
Of course, a lot of the IM's in linux are barely crawling, but a few (amsn, kmess, especially gaim and kopete) are supposed to be functionaing fairly well with font adjustments, color, backgrounds, and one is even supposed to translate practically in communication (real time).
However, as I stated in my help request. There appears to be a serious glitch in the variables of the fonts in kopete and I am hoping someone knows what to do about it. I would especially be interested in knowing if I am perhaps alone with this problem. The way things are going at this moment I do feel quite ALONE with my problem since no one is coming to my assistance.