Welcome! I would have a look through the following two threads to get an idea of the kinds of things you can tweak or steal a couple of snippets to add to your own CSS. The below threads probably have the best information on the subject.
userstyle.css is the file you want to change to have an effect on your rendered markdown (i.e. the markdown viewer) and userchrome.css is the file that you want to change to edit the markdown editor and more or less all of the rest of the application.
In which part of the application was the font colour were you actually attempting to change (i.e. the markdown editor, markdown viewer or richtext editor)?