As said before, Firefox is and has always been a GTK3 app since Firefox 46, which means that editing your .gtkrc-2.0 is unnescesarry. But, the reason Firefox follows your theme and not tor browser is because of this :
Tor-Browser runs with the $HOME
variable set to ~/.tor-browser/app/Browser
instead of just ~/
to be more anonymous. Which means when you run tor-browser, the home directory for it is set to ~/.tor-browser/app/Browser
.
What you can do is create a symlink to your theme :
$ ln -s ~/.themes ~/.tor-browser/app/Browser/.themes
At least, this worked for me, on Arch Linux using the tor-browser package from the AUR.