Extend `theme_importer` in preparation for importing Zed1 themes (#3791)

This PR extends the `theme_importer` with the overall structure required
to support importing themes from Zed1.

Release Notes:

- N/A
dc64411ccaMarshall Bowers committed on 12/22/2023, 6:47:30 PM· committed by GitHubparentc34a811
11 files changedLine totals unavailable