sddm-kcm allows you to install themes which cannot work

[peterhoeg]

Describe the bug
Further to #98997 - sddm-kcm gives you an option to install login themes, but we need the themes to be handled declaratively in order to work.

Ideally we disable the "install theme" button or show some notification.

[stale]

I marked this as stale due to inactivity. → More info

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/cant-setup-global-themes-in-kde/12416/2

[cstroe]

So what would the fix be here? If all configuration in Nixos/Nix is supposed to be declarative, how could a tool like sddm-kcm work?

It seems like sddm-kcm should somehow be patched to be aware of Nixos/Nix and interact with Nix somehow to update configuration. Maybe somehow create an overlay on the system level? Something else?

[cstroe]

So it seems like there was already a discussion about this on #98997, and the recommendation is to have a custom nix script imported into the main config.