My system is Gentoo amd64, updated to the latest. After I updated and restarted my computer, I got this funny transparent background, but only in popup dialogs/menus. I have tried to delete the ~/.kde4 dictionary and restart my computer. But no luck.
OK, it is solved automatically...I have no idea how it happened :(
Probably, it has something to do with the kdewallet. I did not enable the wallet previously, because I do not think it is useful and required. But I could not even connect to my WIFI if it is disabled. So I enabled and configured the KDEWallet, then the background issue resolved :)