In the OS X 10.8 version of Messages on the Mac, you can set the background color of your own messages, and you can set the background color, font, and font color of anyone who sends you messages.
Now, though, in OS X Mavericks, you can do all those things, plus send messages in your own choice of font, as well as let your friends send you messages with their own font choices.
Here’s how to make that happen.