Missing scrollbar styles on nbInput type textarea

### Issue type

**I'm submitting a ...** (check one with "x")

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**

When the content is bigger than rows quantity the scrollbar can't get the theme scrollbar styles.

**Expected behavior:**

That the styles can be applied without any workarround.

**Steps to reproduce:**

Just create an NbInput textarea with 2 rows and add some content to force scroll

**Related code:**
`




``
insert short code snippets here
```

### Other information:

**npm, node, OS, Browser**
`Any version, any browser``

**Angular, Nebular**
`Angular 10.0.10, Nebular 6.0.0 ``

```