* Enable theme context mocking in tests

* Expose mockThemeContext from grafana/ui

* Add docs

* Update contribute/style-guides/themes.md

Co-Authored-By: Marcus Olsson <olsson.e.marcus@gmail.com>

* Update packages/grafana-ui/src/themes/ThemeContext.tsx

Co-Authored-By: Marcus Olsson <olsson.e.marcus@gmail.com>

* Update contribute/style-guides/themes.md

Co-Authored-By: Marcus Olsson <olsson.e.marcus@gmail.com>

* Docs update

* Update contribute/style-guides/themes.md

Co-Authored-By: Marcus Olsson <olsson.e.marcus@gmail.com>