I don't know where you read such a recommendation, but default styles are stored in your Normal.dotm template, not in a plist file. To create a new default Normal.dotm template on your Mac:
- Close Word.
- Hold down the Option key while clicking on the Go menu and choosing Library. Your hidden user library folder opens.
- Navigate to ~/Library/Group Containers/UBF8T346G9.Office/User Content/Templates.
- Rename Normal.dotm to something like NormalOld.dotm.
- Open and close Word. A new Normal.dotm file is created with the styles reset to the current Word defaults.