How to style Firefox Developer Edition (42.0a2) with GTK3 themes

Recently Firefox Dev Edition switched to GTK3 from GTK2 and after the update I was very disappointed to realise that my GTK themes were no longer respected. I am running KDE 4.12.2 on Ubuntu 12.04 with QtCurve theme and have all GTK2+ apps configured to use this theme too. It works nicely and I like the looks.

However, this new gtk3 Firefox having no themes configured looks very ugly. There is a GTK icon in "System settings" which is supposed to let me set the theme for GTK3 as well as GTK2. Alas, it doesn't function / switch themes. There might be a mess-up in my KDE configuration after the years. I simply couldn't make it work.

Playing with GTK3 config files, trying to add GTK_RC_FILES in the environment, create ~/.config/gtk-3.0/settings.ini, /etc/gtk-3.0/settings.ini with theme name "gtk-theme-name = oxygen-gtk". Nothing seemed to work.

Then I decided to check which themes I have compatible with GTK-3 at all:

$ find /usr/share/ -name "gtk.css" 

Then, solution appeared to be very simple after running 'gtk3-widget-factory' under strace ;)

Everything in the directory containing all gtk-3.0 files should be copied to ~/.config/gtk-3.0/

No other setting to change - just place files there and it gets loaded perfectly by Firefox or any other GTK3 app!

Firefox looks good again.