Sidebar partially shown under the header (3.6.2)

[immortaly007]

Issue type

I'm submitting a ...

• bug report
• feature request

Issue description

Current behavior:
In version 3.6.2, the sidebar is shown at a different height (i.e. with a different top value in the CSS).
This causes the sidebar to partially overlap with the header:

This is caused by the following line of code:

nebular/src/framework/theme/components/layout/_layout.component.theme.scss

Line 161 in e5a04b6

top: nb-theme(header-height);

Which calculates the top to be 4.25em (for the default theme), not taking into account the additional 2rem margin for between the header and the content.

Expected behavior:
The sidebar should have some margin with the header, and not overlap (like it used to in 3.6.0 and earlier versions)

Steps to reproduce:

Just add any standard sidebar:
https://stackblitz.com/edit/nebular-sidebar-issue

Related code:

Please refer to my stackblitz:
https://stackblitz.com/edit/nebular-sidebar-issue

Other information:

npm, node, OS, Browser

Node 10.15.3
NPM 6.10.1
OS: Windows 10
Browser: tested in Chrome/Firefox

Angular, Nebular

Angular: 8.1.2
@nebular/theme: 3.6.2 | 3.6.1

The issue doesn't happen in Nebular 3.6.0, so my current workaround is to enforce that version of @nebular/theme