Created attachment 7633 screenshot I set font bold in xfce4-settings-manager, then gtk2 app is right, but gtk3 application is invalid.
This does not appear to be a bug in xfce4-settings. Even using $XDG_CONFIG_HOME/gtk-3.0/settings.ini to set the font does not enable bold or italic. Another report: https://bugs.launchpad.net/ubuntu/+source/gtk+3.0/+bug/1335056