On my Ubuntu 13.04 machine suddenly all GTK2 applications (like Firefox, LibreOffice etc.) are not themed anymore (I am using Ubuntu's Standard theme). Instead, they are shown with this ugly old default theme. I didn't do anything that could have caused that behaviour, as far as I know. Does anybody know what to do?
UPDATE: Maybe a system crash (I had before), that forced me to do a cold reboot, has damaged some system configurations files. But I don't have an idea which one, or how to check.
The screenshot shows the problem: Gedit is themed and Firefox is not. It occurs only when I log in with my user account. With any other everything is ok.