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" /usr/share/themes/HighContrast/gtk-3.0/gtk.css /usr/share/themes/oxygen-gtk/gtk-3.0/gtk.css /usr/share/themes/Clearlooks-Phenix/gtk-3.0/gtk.css /usr/share/themes/Adwaita/gtk-3.0/gtk.css /usr/share/themes/LowContrast/gtk-3.0/gtk.css /usr/share/themes/HighContrastInverse/gtk-3.0/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.