If you are on KDE with openSUSE and want to change the default login theme(by default it is SUSE theme). That will be pretty easy. In case you tried Login Screen in Configure Desktop Settings and its not working, then you can follow this post to have your desired login screen theme. Almost every theme is available on kde-look.org KDM KDE4 themes page, search around and download your favorite one. Lets say you downloaded it to Downloads folder in your home directory, extract it there. [for example this post will use KStarboard-new theme]
Now open terminal and become root by su – , type:
mv KStarboard-new /usr/share/kde4/apps/kdm/themes/
Now open YaST, go to /etc/sysconfig Editor and then click on Desktop -> Display Manager -> DISPLAYMANAGER_KDM_THEME, and you will SUSE or some other value, change it to your theme folder name like KStarboard-new. Save and close YaST. Checkout the screen shot for this.
Reboot your system so that the changes take effect.
If you are editor/terminal lover, you can open the following file in terminal instead of using YaST and edit it there(via root user):
Look for this line [DISPLAYMANAGER_KDM_THEME=”SUSE”] and add your theme folder name there.
That should do it.