I figured it out:
I looked in the .xsession-errors file and found out that
XfcePanelWindow-popup-delay and the other property names are not valid property names. Then I looked carefully again in
how the gtk.css file in Ubuntu was worded and it turns
out that each property name must have a - in front of it to be valid. In other words, instead of
XfcePanelWindow-popup-delay
it must be
-XfcePanelWindow-popup-delay
This solved it and now it works.