there is no
file...
it's a stylesheet in the
database, which you could see in the admin panel under 'layout > stylesheets'
but i must admit.... i just installed all versions from 5.5 (which was the one i used recently) to 6.2 and there is no stylesheet
strange....
well, the stylesheet was crappy anyway, it's the easiest to make a new one.
you can see the classes and id's available with firefox addOns like webdeveloper toolbar or firebug.
good luck... sorry, I can't give you a default stylesheet.....