Add CSS [How to Format Epub Files within Scrivener]

I seem to recall that there’s a place where you can include additional css when compiling.
Where is this section, or did I imagine it?

(two days later) Wow, I have never before stumped the panel. Since CSS is what governs text layout in e p u b files, you would think that this would be of interest. Apple’s Pages uses a generic CSS stylesheet but apparently it is hidden. The Open Office plug-in Writer2epub doesn’t let you modify CSS either. Sigil does, but it’s a pain to use, especially with a document compiled with Scrivener (Scrivener creates multiple CSS stylesheets–do you have to edit one? Will deleting all but one affect the text? Who knows?) Maybe Calibre is the way to go, but there’s still that nagging multiple CSS question…