Figured this one out... Ubuntu's /etc/gdm/Xsession doesn't set SYSRESOURCES (and USRRESOURCES) before sourcing /etc/X11/Xsession.d/30x11-common_xresources, so /etc/X11/Xresources/x11-common is never read.
Figured this one out... Ubuntu's /etc/gdm/Xsession doesn't set SYSRESOURCES (and USRRESOURCES) before sourcing /etc/X11/ Xsession. d/30x11- common_ xresources, so /etc/X11/ Xresources/ x11-common is never read.