docs: Document recipe on how to implement theme switching

[MartinCupela]

Motivation

With the introduction of theming V2 will not intend to maintain theme provider logic and this should be implemented by the integrator. What we however will provide is a recipe in documentation to make the users familiar:

1. What dark / light themes the theming v2 distinguishes
2. How to override theme CSS variables
3. How to provide custom theme
4. How to implement a simple theme provider