Hey Lutz,
make changes in ie6.css file which will be available under Content Administration---Web Resource Repository---Portal Content---Web Resources---theming---Portal---AFP---<Theme Name>.
Under this folder many css files exists. Download the ie.css file and make required changes in it and then upload in the same path by overwriting the existing one.
The css class which you've mentioned will be available in the above css file.
Regards,
Kamal
