* What led up to the situation?
I changed the default theme for a moment, then I couldn't come back to the previous GTK 3.0 theme, because there's no option for it, since all themes that only have a GTK 2.0 rc file are not available for selection any more.
I managed to work around it editing the xsettings.xml file and deleting all GTK 3.0 themes
I left this line like so:
<property name="ThemeName" type="string" value="Default"/>
In ~/.config/xfce4/xfconf/xfce-perchannel-xml/xsettings.xml
Restarted to the session and got the default theme back. However, this Default theme had no GTK 2.0 folder, so I copied the one that's on /usr/share/themes/Xfce/ and everything went back to normal.
I still can't select the Default theme in the Apparience Menu.
It would be ideal to add a Default entry in this menu, that combines the default GTK 3.0 theme, and the default Xfce GTK 2.0 theme (for GTK 2.0 applications).
Thank you for your time.