vivid: modify themes option with custom nix entries

[arunoruto]

Description

The current themes option only allows for downloading external YAML themes, and not for generating custom ones with Nix code. This will be useful when styling NixOS and home-manager uniformly with stylix, but nonetheless, it is a helpful option to have, in my opinion.

@aguirre-matteo would be glad to get some feedback on this!

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

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  nix-shell --pure tests -A run.all.

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[aguirre-matteo]

IMO adding another option seems redundant. In any case, we could change the actual themes option to be an attribute set of either a path or yamlFormat.type, and then map cfg.themes to generate the YAML files.

[arunoruto]

IMO adding another option seems redundant. In any case, we could change the actual themes option to be an attribute set of either a path or yamlFormat.type, and then map cfg.themes to generate the YAML files.

Fair enough! I guess the current changes make more sense?