切换图标未出现在Bootstrap 4中

Question

我正在尝试实现自举导航栏，并且试图将navbar-toggler-icon（类似于汉堡包）设置为白色，但是不起作用。将深深感谢您的帮助。下面的照片和代码

image of burger icon that toggles on contracting the page

html代码

<nav class="navbar navbar-expand-lg navbar-inverse navbarcustom">
    <div class="container">

  <a class="navbar-brand" href="#">  <i class="fa fa-picture-o" aria-hidden="true"></i> ART</a>

  <button class="navbar-toggler text-white" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>

  <div class="collapse navbar-collapse" id="navbarSupportedContent">
    <ul class="navbar-nav mr-auto">
      <li class="nav-item active">
        <a class="nav-link" href="#">About <span class="sr-only">(current)</span></a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Contact</a>
      </li>

    </ul>
    <a class="nav-link mr-sm-2" href="#">Sign Up</a>
    <a class="nav-link mr-sm-2" href="#">Login</a>

  </div>
  </div>
</nav>

css文件

.navbarcustom{
    background-color: #5248a3;

}
.navbar-light .navbar-brand {
  color: white;
}
.white{
    color: white;
}

nav a{
    color: white;
}
body{
    font-family: 'Zilla Slab', serif;
}
.jumbotron h1{
    font-size: 60px;
}
.jumbotron p{
    font-size: 30px;
    font-weight: normal;
}
i{
    color: white;
}
h1 i{
    color: black;
}
button span{
    color: white;
}
.navbar-toggler-icon {
  display: inline-block;
  width: 1.5em;
  height: 1.5em;
  vertical-align: middle;
  content: "";
  background: no-repeat center center;
  background-size: 100% 100%;
  color: white;
}

预先感谢