.icon-stack {
    position: relative;
    display: inline-block;
    width: 2em;
    height: 2em;
    line-height: 2em;
    vertical-align: middle;
}
.icon-stack-1x,
.icon-stack-2x {
    position: absolute;
    left: 0;
    width: 100%;
    text-align: center;
}
.icon-stack-1x {
    line-height: inherit;
}
.icon-stack-2x {
    font-size: 2em;
}
.icon-inverse {
    color: #ffffff;
}