Debian is not known for its instability and things do change when in stable they are mitigated. In testing things do get funky and sddm (a command) is your new kdm/gdm thing.
The theme files can mess you up if you delete one and still have it set up in user preferences (remember x does not do root anymore)
So themes need a bit a thought before you delete them. Overal running debian testing is a choice i am willing to make as it makes debian stable upgrades a lot easier (my blog) when i upgrade the important servers.
sddm is a bit rough round the edges running gui tools like kate as root seems to be a impossible thing when i write this, but sddm can be switched off as despite being not recommended a window manager can be usefull for root when you have lots of text files to edit. No i dont do it often but sddm is very much for users rather than administrators with servers to administer.
I am sure there are fixes for those of us who while we like vi also like kate, so it has limitations for some of us but xserver-xorg-legacy might be needed by some.