input[type='checkbox'] {
    width: 14px !important;
    height: 9px !important;
    margin: 5px;
    -webkit-appearance: none;
    -moz-appearance: none;
    -o-appearance: none;
    appearance: none;
    box-shadow: none;
    font-size: 0.8em;
    text-align: center;
    line-height: 1em;
    background: white;
}

    input[type='checkbox']:checked:after {
        content: '✔';
        background-color: white;
    }