Hi Adisa,
themes that have been installed as plugins cannot be modified. This is intentional.
If you want to update such a theme using gulp there are two options:
- Use the user interface to create a copy of the theme. The copy can be modified using the online editor as well as the REST API / gulp. You
may uninstall the original theme (the plugin) afterwards to get rid of the duplicate.
- Extract the jar file instead of installing it as a plugin and then upload the extracted files to a new theme that you've created for
example using the user interface.
The purpose of the JAR / plugin theme download option is to create a snapshot out of a theme in your development / testing environment and
then upload the plugin to your production environment once the theme development is complete.
Hope that helps.
Cheers,
Jens