helix: add support for path and string themes

[0xda157]

Description

The waybar module has similar functionality. Adding this because stylix team would like to be able to use a string/path for the theme as disused in nix-community/stylix#1125 (comment)

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all
  or nix build --reference-lock-file flake.lock ./tests#test-all using Flakes.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.

Maintainer CC

@Philipp-M

[0xda157]

hmm seems to be broken for multiple reasons, currently working on fixing it.