Just one line of coding is enough.. just try this out. and you can adjust even thicknes of icon-bar with this by adding pixels.
HTML
<div class="navbar-header">
<button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#defaultNavbar1" aria-expanded="false"><span class="sr-only">Toggle navigation</span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
<a class="navbar-brand" href="#" <span class="icon-bar"></span><img class="img-responsive brand" src="img/brand.png">
</a></div>
CSS
.navbar-toggle, .icon-bar {
border:1px solid orange;
}
BOOM...