In Gentoo and Kubuntu GNU/Linux, when I want to add a theme for the KDE login manager (KDM), I can select and install new themes, but they are not added to the list of installed themes. Here’s a quick workaround.
I access the settings through the menu: Start (button) -> Computer (tab) -> System Settings (button) -> System Administration (section) -> Login Screen (button) -> Theme (tab)
When pressing the Get New Themes button, I can browse, select and install themes. However, when returning to the list of available themes, the newly installed theme is not listed.
This turns out to be a well-known, but apparently never fixed bug. The problem seems to be that root permission is only sought after downloading the new theme, which means that the download cannot be saved.
Until this is fixed, a workaround is giving everybody permission to save the file. This can be done on the command line by typing:
Gentoo (as root):
chmod a+w /usr/share/apps/kdm/themes
sudo chmod a+w /usr/share/kde4/apps/kdm/themes
(or by performing a smiliar action (making the directory world writeable) in a file manager – you’ll still need root permissions).