Ian,
Sadly, for GP 2013, CSS changes do not effect the home page. In fact, you can delete the CSS and your Home Page is still happy.
I have modified the CSS for MANY of my clients' Home Pages, but now - poof - they are gone. The clients are not happy with this development. I think I'll put a suggestion in on Connect.
If you know of a way to edit the GP 2013 Home Page, please let us know :)
Kind regards,
Leslie