I can’t see a way of Restoring (or Applying) the default Paragraph Styles in an existing document. (I’ve actually just been clicking on Restore to Factory, and then realised it’s all gone Academico…!)
Is there such a thing?
When you say “default”, I guess you mean the styles you have saved as default that will be used when you start a new project, rather than back to the factory defaults? If so, then the answer is no: you can only revert back to the factory defaults. (It’s true that you can do this with the various options dialogs, but not with library items.)