Unable to apply nebular styles to any component

### Issue type

**I'm submitting a ...**

* [ X] bug report
* [ ] feature request

### Issue description

**Current behavior:**
I have an existing project in which I want to add nebular components.

I have successfully added a stepper, but none of the Nebular styles are applied to any element of the stepper (including buttons).

I have followed the instructions on how to integrate the styles, but it is not working as expected. The bootstrap styles do work as expected.

I've also tried to manually add the bundled css file to index.html, but it does not work anyways.

Finally, I've also started from scratch with the ngx-admin starter app, and I've imported all stylings from that project (where styling was working). Still Nebular components are not styled in the project in which I would like to integrate Nebular.

**Expected behavior:**
Following the step-by-step guide on how to integrate Nebular should result in the styles properly applied.

**Steps to reproduce:**
1. Start a new project with `ng new my-project`. Add Bootstrap and NgBootstrap.

2. Import all bootstrap style as scss in a `styles.scss` file, and add such `styles.scss` file to `angular.json`.

3. Follow the step by step guide on how to integrate Nebular in an existing project.

**Related code:**

[https://stackblitz.com/edit/angular-nebular-integration](https://stackblitz.com/edit/angular-nebular-integration)

I've added the stepper and the button, and none of the elements is properly styled.

### Other information:

See the stackblitz example.

**Angular, Nebular**

v. 2.0.0-rc.10