[HOTFIX] Change active theme for preview

* Change active theme in preview
* Fixed loader bugs in content form
* Fixed special case for single block

__Tasks:__

- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | #185, #179 
| BC Breaks     | none
| Doc           | none