The reason your CSS isn't working is because of specificity. The Bootstrap selector has a higher specificity than yours, so your style is completely ignored.
Bootstrap styles this with the selector: .navbar-default .navbar-toggle .icon-bar
. This selector has a B specificity value of 3, whereas yours only has a B specificity value of 1.
Therefore, to override this, simply use the same selector in your CSS (assuming your CSS is included after Bootstrap's):
.navbar-default .navbar-toggle .icon-bar {
background-color: black;
}
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