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.
Maybe you also know why the image on the login page does not change despite I added a custom logo-icon.svg. It works after login. According to the doc it’s the same image:
The logo at the login-page above the credentials-box General owncloud/core/img/logo-icon.svg
The logo in the left upper corner after login owncloud/core/img/logo-icon.svg
when updating the various logo files (logo-icon, logo etc), the one on the login page (logo.svg) does not change, only the one after login. However on the sample theme it does work. So I wonder if there is something specific about the .svg file.
I found the issue with the logo not updating. In your theme you must have styles.css & header.css as well otherwise the custom logo is not loading. Of course, this also means you need to copy all the other images as well as the path otherwise does not match.