select2/docs/_sass/_hamburger.scss

/* Animated hamburger icon @see http://codepen.io/JulienMelissas/pen/LEBGLj */
.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%;
  }
  &.collapsed .top-bar {
    transform: rotate(0);
  }
  &.collapsed .middle-bar {
    opacity: 1;
  }
  &.collapsed .bottom-bar {
    transform: rotate(0);
  }
}