<button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar" aria-expanded="false" aria-controls="navbar">
  <span class="sr-only">Toggle navigation</span>
  <span class="icon-bar top-bar"></span>
  <span class="icon-bar middle-bar"></span>
  <span class="icon-bar bottom-bar"></span>
</button>

CSS

navbar-toggle {
  border: none;
  background: transparent !important;

  &:hover {
    background: transparent !important;
  }

  .icon-bar {
    width: 22px;
    transition: all 0.2s;
  }
  .top-bar {
    transform: rotate(45deg);
    transform-origin: 10% 10%;
  }
  .middle-bar {
    opacity: 0;
  }
  .bottom-bar {
    transform: rotate(-45deg);
    transform-origin: 10% 90%;
  }
}

.navbar-toggle.collapsed {
  .top-bar {
    transform: rotate(0);
  }
  .middle-bar {
    opacity: 1;
  }
  .bottom-bar {
    transform: rotate(0);
  }
}