Vertical scrollbar not visible 🐛

### Issue type

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

* [x] bug report
* [ ] feature request
* [ ] question about the decisions made in the repository

### Issue description

**Current behavior:**
Using your template I noticed a strange behavior.
When there are many things to display on the page the vertical scroll bar remains under the fixed header. This is very annoying because if a user has a page with several elements arranged vertically he cannot view the scroll bar because it is below the header.

**Expected behavior:**
I would expect to see the vertical scroll bar immediately after the end of the fixed header and not below.

**Steps to reproduce:**
It can be reproduced on your pages as:
[https://www.akveo.com/ngx-admin/pages/layout/infinite-list
](https://www.akveo.com/ngx-admin/pages/layout/infinite-list
)
**Related code:**
To highlight it you can change the color of the scroll bar

```
.nb-theme-default nb-layout ::-webkit-scrollbar-thumb {
background: #005bff;
cursor: pointer;
border-radius: .15625rem;
}

```