Consider allowing custom CSS updates in the UI

[laryn]

Follow up from #82

Comment copied from the issue above:

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:

• Allow custom css via user interface pelerine#5