Custom SDDM theme in nixos?

Help
1

How do i apply a custom sddm theme in nix? i can’t copy the sddm theme folder into /run/current-system/sw/share/sddm/themes/ since its read-only. I wanted to apply the Ant-Dark sddm theme. Neither can i copy it into the folder nor can it be configured in kde system settings.

tried doing it but:

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2

When you interpolate an expression, it must generally evaluate into a string, a path or a derivation.

What is inside sddm-theme.nix? It looks like it is a function that returns a set.