DEV: Compile core and plugin stylesheets independently of themes (PR #13638)

Take 2 of DEV: Compile core and plugin stylesheets without themes by pmusaraj · Pull Request #13466 · discourse/discourse · GitHub.

Fixes a few issues with the original PR:

  • color definition stylesheet target now includes the theme id, to avoid themes set to use the default color scheme loading the same stylesheet
  • changes the internal cache key for color definition stylesheet to reset the pre-existing cache

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