One simplest way to encounter this is to use font awesome for example
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarCollapse" aria-controls="navbarCollapse" aria-expanded="false" aria-label="Toggle navigation">
<span><i class="fas fa-bars"></i></span>
</button>
Then u can change the i
element like you change any other i
element.