This is just a note to think about: I'm trying to think about feature-parity with the Drupal version, and the custom theming piece linked above. CSS Injector is one alternative, but rather than require a third-party module for that, I wonder if it would make sense to implement something like what Pelerine has done with custom CSS in the UI:
Follow up from https://github.com/backdrop-contrib/gin/issues/82
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