Enhancement: Initialize the design system with themable colors

By passing suitable plugin options, you can overwrite the CSS color variables within the ODS to adjust it to your likings.
The default color values are generated from within the design system and act as a fallback 
if no (or not all) options are passed when initializing the design system plugin.

https://github.com/owncloud/owncloud-design-system/pull/1168
