Pure CSS nav-toggle icon

styles.css

html {
  font-size: 17px;
}

@base: 1rem;

.nav-toggle {
  display: block;
  position: relative;
  top: 0;
  left: 100%;
  z-index: 999;
  margin-left: -@base*3;
  background: @white;
  height: @base*3;
  width: @base*3;
  font-size: 0;
  &:before {
    content: "";
    position: absolute;
    top: 50%;
    left: 50%;
    margin: -10px 0 0 -((@base*3)/3.14)/2;
    width: @base*3/3.14; // pi = golden
    height: 3.2px;
    color: @dark-grey;
    border-top: 9px double;
    border-bottom: 3px solid;
  }
}
}