This plugin shows how to set document templates.
It is called "Document Templates" in the interface and isn't installed by default in cloud, self-hosted and desktop version of ONLYOFFICE editors.
- Find the plugin in the Plugins tab.
- Сhoose any style you want to set.
If you need more information about how to use or write your own plugin, please see this https://api.onlyoffice.com/plugin/basic