Kubuntu 18.04 Tor Browser don't use gtk theme set by .gtkrc-2.0

I installed torbrowser-launcher version 0.2.9-2 which provided me with the Tor browser which itself is based on Firefox 60.3.0esr:

The point to note is that Firefox is and has been a gtk3 application since Firefox 46. So your gtkrc-2.0 settings will not be effective.

I suggest you look at System Settings > Application Style > GNOME Application Style and make a suitable choice there.

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.