Whenever I would run a gtk3 application, I would get an error message:

"
Gtk-WARNING **: Theme parsing error: gtk.css:72:18: Not using units is deprecated. Assuming 'px'.

Gtk-WARNING **: Theme parsing error: gtk.css:72:20: Not using units is deprecated. Assuming 'px'.
"

Upon tracking down gtk.css first to /usrshare/themes/Xfce-dusk/gtk-3.0 I found that changing line 72 from 
"text-shadow: 1 1 white;"
to
text-shadow: 1px 1px white;
fixed this incredibly annoying error message.

Cheers.