Angular Nebular styles does not apply for NbChatComponent

### Issue type

* bug report
* [ ] feature request

### Issue description
I have an ongoing Angular project and I'm trying to add Nebular Chat UI to the project.

I installed nebular with npm and did the imports as mentioned in the site. Functionality is working as expected, but the styles are not applied to the component.

Below are the steps I followed.

```
npm install --save @nebular/theme @angular/cdk @angular/animations
npm install --save @nebular/eva-icons
```
Imported NbThemeModule and NbChatModule in app.module.ts
```
import { NbThemeModule, NbChatModule } from '@nebular/theme';

@NgModule({
imports: [
...

NbThemeModule.forRoot(),
NbChatModule
]
```
Added styles in angular.json
```
"styles": [
"node_modules/@nebular/theme/styles/prebuilt/default.css"
]
```
Added html component (Sample available in the site)
```

```
And get this result:

here is a link to [stackoverflow](https://stackoverflow.com/questions/61104967/angular-nebular-styles-does-not-apply-for-nbchatcomponent/61408369#61408369)

**Related code:**

here is a link to [codesandbox](https://codesandbox.io/s/nebular-chat-example-p398q)

### Other information:

**npm, node, OS, Browser**
```

```

**Angular, Nebular**
```

```