If you are talking about for a particular document, you can use the organiser (Organiser... button from your screenshot) to copy styles between the style-set for the current document and the global defaults stored in Normal.dotm.
According to the article in the link below, the Normal.dotm must exist and when deleted, Office restores it. So to restore global defaults, the suggestion of deleting Normal.dotm seems like a logical solution. Simply:
rm ~/Library/Application\ Support/Microsoft/Office/User\ Templates/Normal.dotm from the terminal works.