Toggle Sidebar Button Rendered Ugly

[n-ae]

Issue type

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

• bug report
• 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

    <a (click)="toggleSidebar()" href="#" class="navigation"><i class="nb-menu"></i></a>

Other information:

npm, node, OS, Browser

<!--
Node, npm: v10.5.0 and 6.1.0
OS: Windows 10
Browser: Chrome/Firefox
-->

Angular, Nebular

<!--
Angular: 6.0.0
Nebular: 2.0.0-rc.9
-->

[n-ae]

Right now the most consistent icon among the three(nb, ion, font-awesome) seems to be the font-awesome one. As in:
<a (click)="toggleSidebar()" href="#" class="navigation"><i class="fa fa-bars"></i></a>
which doesn't exactly perfectly fitting(It's a bit too large).