In MDL-78425/MDL-78426 we implemented a new theme selector/settings UI, however the implementation did not factor in that some themes had multiple settings pages/screens, so the new functionality only supported one. The result was that we needed to reinstate the "old" theme category as "Custom theme settings" to both avoid errors and make it possible to access any additional theme settings pages required by the themes.

Requirements for this issue:

1. Implement a way to support multiple theme pages in the "new" theme UI (this will require UX input).
2. Remove "Custom theme settings" since it will no longer be required. TBC: Whether this is immediately removed (with notice provided to theme designers with any code changes required to be compatible), or hidden and deprecated to avoid any errors, but remove the old UI.