There are two things you can style: the source code display, and the interactive view.
Source Code ===========
First, create a CSS file setting what you want - for example, you can "make the text smaller" <code-smaller.css>.
Second, tell JSD to use this "motif" (file that controls how it looks). You use the preference "services.source.css" for this, so you would save the above CSS file, or write your own, and open it up in the browser so you have a file: URL. Then enter (into the interactive view):
followed by the file: URL from the browser. You can use the CSS sample above, if you like, by doing: