Adjusted to new theme config

Adjusted the bundle to the new theme config (themes are configured at workspace level now)
Merge after https://github.com/sulu-cmf/sulu/pull/26

- [x] test coverage
- [ ] finish the code
- [x] gather feedback for my changes
- [x] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| Tests pass?   | yes
| Fixed tickets | yes
| Doc           | none