Public Theming API

[MariusBluem]

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: nextcloud/android#213

cc @juliushaertl @AndyScherzinger @Bugsbane @nextcloud/designers

[Bugsbane]

Totally agree. This is necessary for consistency of branding, especially for more enterprise style use.

[MorrisJobke]

Implemented in #1272