Toggle Sidebar Button Rendered Ugly

### Issue type

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

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

### Issue description

**Current behavior:**

The toggle button for sidebar seems to be rendered non-definitively and looks ugly in my opinion.
**Expected behavior:**

I'd expect the button to have a more consistent image as in at least having some kind of symmetry if making all three lines equal is a headache.
**Steps to reproduce:**

It is visible in the current demo version. In 100% zoom(no zoom). 1920x1080 screen resolution.
**Related code:**

https://github.com/akveo/ngx-admin/blob/master/src/app/%40theme/components/header/header.component.html
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### Other information:

**npm, node, OS, Browser**
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**Angular, Nebular**
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