help with AO3 issue, please?

Technical question from a person who knows possibly less than nothing about computer coding:

How do you make the AO3 display different font faces and colors within the text of a story? I know this is possible because I've seen other people do it in their stories. But the site doesn't recognize the old html font code, and the only thing I know about CSS is that it is called CSS and has something to do with websites and display and stuff, full stop.

I need this trick to make the chatlog portions of my Homestuck porn fic display correctly.

Thank you for any help!