NbList having default scrollbar

### Issue type

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
ng-list component has default(standart) scrollbar appearance

**Expected behavior:**
ng-list component to have custom(defined by selected theme) appearance

**Steps to reproduce:**
1. git clone https://github.com/akveo/ngx-admin.git
2. cd ngx-admin && npm i
3. npm start
4. locate to nb infinite list tab

### Other information:

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

```

While visiting demo of nebular at https://www.akveo.com/ngx-admin/pages/layout/infinite-list
shows proper scrollbar stylement.
Cannot find any calls to scrollbar mixin in @nebular/theme/components/nb-list
There is just no styles like:
````css
.nb-theme-default nb-card nb-list::-webkit-scrollbar-thumb
````
, in browser. On a clean install.