Default theme

This commit creates a default theme which will help users to get started with theming. A default theme is mentioned in the documentation although no default theme exists, this would fix that issue and would prevent us from having to make changes to the documentation.

This issue has been mentioned here https://github.com/owncloud/core/issues/12376 https://github.com/owncloud/core/issues/9627 https://github.com/owncloud/documentation/issues/425

This commit allows a user to easily, among other things, modify the name of their ownCloud installation. It also provides a basic starting point showing where their style.css file needs to be to make basic changes to styles.