9
You can do this ...
{composition-setup}import.css=/download/attachments/123456789/custom.css
{composition-setup}
That's if you've stored a custom.css file as an attachement. You'd obviously need to replace 123456789 with the actual attachment number.
You can also link CSS on an external site (with an absolute URL), but if you have any automatic URL formatting, that tends to mess it up everytime you change the document.
I use a User Macro that renders the $body in HTML. Then I can put whatever HTML tags I want in the wiki page within the user macro tag.