Version: (using KDE Devel) Installed from: Compiled sources kdm should include a theme testing program to allow artists to quickly test their themes without starting up a new X session. It would also make it much easier to take screenshots. Using kdmtheme from kdenonbeta (now removed) works fine. gdm also includes gdmthemetester which is a shell script that uses Xnest. Either would work great included with kdm.
*** Bug 94415 has been marked as a duplicate of this bug. ***
I'm not a KDE developer, but have a workaround for this issue: Running "Xnest -query localhost :1" opens a login screen if kdm has XDMCP enabled. The same trick can also be done with "vncviewer :1" if you have a Xvnc running through xinetd.
gdmthemetester does about the same, except that it makes gdm start an Xnest itself (so no xdmcp is required) - but this is currently not supported by kdm. the other requirement would be being able to tell kdm to use a certain theme without changing the config file (you don't want the setting to persist if the system crashes). oh, btw, have i already mentioned that themes are not officially supported, yet? :)
KDM is unmaintained and not used in KDE Plasma 5. SDDM is the login manager used in KDE Plasma 5. If you still have this same issue with SDDM, please file an issue on the SDDM bugtracker (after doing a search for existing issues first!): https://github.com/sddm/sddm/issues/