I created a custom theme based on the documentation: https://doc.owncloud.com/server/10.0/developer_manual/core/theming.html
According to this doc:
You only need to sign your theme if you are going to publish it as an app in the marketplace. If you are only creating a private theme for your own ownCloud installation, then you can get rid of the signature file.
However this throws several integrity check errors.
Using latest version 10.0.4