I have been working on building a custom theme for Owncloud 10.0.4 and noticed the document is lacking some info. For example: to change the logo on the login page, copying your own logo.svg to /theme-something/core/img is not sufficient. I only got this to work by also copying styles.css and header.css to /theme-something/core/css
This also required me to copy all the other images due to the relative path usage in the css files.
I'm not sure if this is how it was intended to work. If not, it's a bug, else the document could go a bit more in detail on this.