SUMMARY
Currently, barring using /etc/skel, there is no way to set the default GTK theme for downstream Linux distributions. In my case, Ubuntu Studio, we use the Materia theme by default. We wish to make it the default for GTK and Qt applications, which I can do for Qt, but not for GTK.

STEPS TO REPRODUCE
Attempt to set default GTK theme via $XDG_CONFIG_DIRS/gtk-3.0/settings.ini and/or $XDG_CONFIG_DIR/.gtkrc-2.0


OBSERVED RESULT
Settings are completely ignored, defaults to Breeze theme.

EXPECTED RESULT
Settings default to theme set in $XDG_CONFIG_DIRS/gtk-3.0/settings.ini and/or $XDG_CONFIG_DIRS/.gtkrc-2.0

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Ubuntu 20.04 LTS
(available in About System)
KDE Plasma Version: 5.18.4
KDE Frameworks Version: 5.68.0
Qt Version: 5.12.8

ADDITIONAL INFORMATION
I'm the Ubuntu Studio lead, and, while not completely barring our transition to KDE Plasma, this does create an issue for users that hit the "default" button and it defaults to Breeze. Currently, I'm having to use /etc/skel to set the default for new user accounts, which is far from ideal.