From a downstream perspective, when shipping multiple themes, it would be awesome to have themes install a config file to /usr/share/sddm/config.d to override the default theme from /etc/sddm.conf

Alternatively, maybe implementing a fall back mechanism would make sense as well. For eg. Try to load theme from /etc/sddm.conf , if theme doesn't exist check /usr/share/sddm/conf.d