Since some weeks i noted that oxygen (widget) style has a bug/feature that prevents applications paint a background on, at least, kdialogs/qdialogs/qframes by applying a qstylesheet style. This makes login window and password dialog to see ugly.
I noted this since kde version 3.97. Thats the reason i still use kde 3.96 on my computer. Of course all screenshots and test were done with kde 3.96.
I told this on #oxygen channel at irc.freenode.org, but got no responses. I will try again later.
UPDATE: I was told that with kde 4.0.1 the bug is gone.
UPDATE: I try myself kde 4.0.1 and the bug/feature is still present.