So the SDDM theme (breeze) imports the breeze dark colors, that is in a lock screen. When I login though, I would like the inputs to also be dark as they should be if I am using the breeze dark theme. Also, even if I add XDG_CURRENT_DESKTOP="KDE" to /etc/environment and go in the settings as root and choose breeze dark the problem is still there.
KDM is obsolete, and SDDM bug reports are tracked at https://github.com/sddm/sddm. However this request is for the Breeze SDDM theme, so moving it there.
As a workaround you can copy the relevant config files from ~/.config into /var/lib/sddm/.config SDDM can't use your user theme prferences because by definition you're not logged in yet so we don't know what user we are, and we can't read stuff in your home directory at that point. I'm not sure I'll ever fix this; Duplicating every setting into the SDDM config isn't a good idea...and having everything else sync (including root password prompts and coyping files) is also a bad idea. I'll look at special-casing breeze dark. as a second SDDM theme, but that's about it.
I suppose a separate theme would be the best solution.
This is possible since 5.17 with the sync settings feature in the SDDM kcm.