Public Theming API

We need a public API for theming 🎨 ... This would make it possible that external apps can use the theme values. We've already discussed the use-case (e.g. for the android app) here: https://github.com/nextcloud/android/pull/213

cc @juliushaertl @AndyScherzinger @Bugsbane @nextcloud/designers 
