how to make private message unread number red

[Kalina]

I'd like to take this line;

Private Messages: Unread 0, Total 27.

and make it like this when there's new messages:

Private Messages: Unread 1, Total 27.

I want to make the unread number bold and red when there's new messages.

Is there a way to do this only editing the theme files?

Thanks.