.bt-menu.bt-menu-open {
  height: 100%;
}

.bt-menu-trigger {
  position: fixed;
  top: 15px;
  left: 20px;
  display: block;
  width: 50px;
  height: 50px;
  cursor: pointer;
}

.bt-menu-trigger span {
  position: absolute;
  top: 50%;
  left: 0;
  display: block;
  width: 100%;
  height: 4px;
  margin-top: -2px;
  background-color: #000;
  font-size: 0px;
  transition: background-color 0.6s;
}

.bt-menu-open .bt-menu-trigger span {
  background-color: transparent;
}

.bt-menu-trigger span:before,
.bt-menu-trigger span:after {
  position: absolute;
  left: 0;
  width: 100%;
  height: 100%;
  background: #000;
  content: "";
  transition: transform 0.6s;
}

.bt-menu-trigger span:before {
  transform: translateY(-250%);
}

.bt-menu-trigger span:after {
  transform: translateY(250%);
}

.bt-menu-open .bt-menu-trigger span:before {
  transform: translateY(0) rotate(45deg);
}

.bt-menu-open .bt-menu-trigger span:after {
  transform: translateY(0) rotate(-45deg);
}