You can try the code I posted above in the Calibre Extra CSS field, see if it works. It may not, especially if there is already a p tag in the stylesheet.
I emailed Kobo support about the problem with italics displaying as bold in epubs for some of their fonts, e.g. Kobo Nickel. Their response was essentially I should use kepub instead, as this is their main platform, and the fonts work OK in kepubs. They do not seem interested in fixing the fonts for epubs.
Last edited by ShellShock; 04-27-2013 at 08:36 AM.