input[type=checkbox] {
    position: relative;
    cursor: pointer;
}

    input[type=checkbox]:before {
        content: "";
        display: block;
        position: absolute;
        width: 16px;
        height: 16px;
        top: 0;
        left: 0;
        border: 2px solid #555555;
        border-radius: 3px;
        background-color: white;
    }

    input[type=checkbox]:checked:after {
        content: "";
        display: block;
        width: 5px;
        height: 10px;
        border: solid black;
        border-width: 0 2px 2px 0;
        -webkit-transform: rotate(45deg);
        -ms-transform: rotate(45deg);
        transform: rotate(45deg);
        position: absolute;
        top: 2px;
        left: 6px;
    }