Bitnami ownCloud Cloud Image is set taking security in mind. That's the reason why we set very strong permissions to some folders.
When you log in your server using the Google Cloud SSH web client you are logged as user "YOURNAME". However the user who owns the ownCloud directory and subfolders is the user "bitnami". Could you try to log using the Google Cloud SSH web client and then execute?
sudo su bitnami
After that you will be logged as user "bitnami" and you should be able to access the theme folder.