Currently, when setting 'XfceNotifyWindow::summary-bold = 1' for a theme the summary font type and size will revert to the system's one. So if we set a different font and font size for the body, the summary won't use those settings if it's set to use bold.
So either there should be a separate font_name settings for the summary (and then maybe just drop the summary-bold option altogether) or it should use the font settings for the body (text) when set to use bold.
See with bold on: http://en.zimagez.com/zimage/screenshot-060212-102242.php
And with bold off: http://en.zimagez.com/zimage/screenshot-060212-102315.php
Fixed in git master 6f45653e6cd519327e7166b8b3f3b5e3b982fc42.
summary-bold is now removed and the summary and body widgets have their own widget name ("summary" and "body") which allows you to theme them independently.
Thank you for your bug report.