Fix a colour for checkbox if condition is met

I have seat plan for a hall and I am trying to create a seating booking system. How do I turn the checkbox red if a condition is met i.e. the seat is taken? Also, how do I make it stay red?

My seating plan and a box marked green when checked

Below is a snippet of my code for row A.

                     <!--A-->
                     <tr>
                         <td>A</td>
                         <td></td>
                         <td></td>
                         <td></td>
                         <td></td>
                         <td></td>
                         <td>
                             <input type="checkbox" class="seats" name="selected[]" value="A26">
                         </td>
                         <td>
                             <input type="checkbox" class="seats" name="selected[]" value="A25">
                         </td>
                         <td>
                             <input type="checkbox" class="seats" name="selected[]" value="A24">
                         </td>
                         <td>
                             <input type="checkbox" class="seats" name="selected[]" value="A23">
                         </td>
                         <td>
                             <input type="checkbox" class="seats" name="selected[]" value="A22">
                         </td>
                         <td>
                             <input type="checkbox" class="seats" name="selected[]" value="A21">

This is the relevant CSS code.

.smallBox::before {
    content: "";
    width: 15px;
    height: 15px;
    display: inline-block;
    margin-right: 10px;
}

.greenBox::before {
    content: "";
    background: Green;
}

.redBox::before {
    content: "";
    background: Red;
}

.emptyBox::before {
    content: "";
    box-shadow: inset 0px 2px 3px 0px rgb(255, 152, 0), 0px 1px 0px 0px rgba(255, 255, 255, .8);
    -moz-box-shadow: inset 0px 2px 3px 0px rgb(255, 152, 0), 0px 1px 0px 0px rgba(255, 255, 255, .8);
    -webkit-box-shadow: inset 0px 2px 3px 0px rgb(255, 152, 0), 0px 1px 0px 0px rgba(255, 255, 255, .8);
    background-color: #fff;
}

.seats {
    border: 1px solid red;
    background: yellow;
}

.seatGap {
    width: 40px;
}

.seatVGap {
    height: 40px;
}

input[type=checkbox] {
    width: 13px;
    margin-right: 14px;
    cursor: pointer;
}

input[type=checkbox]:before {
    content: "";
    width: 25px;
    height: 18px;
    border-radius: 5px;
    /* Safari 3-4, iOS 1-3.2, Android 1.6- */
    -webkit-border-radius: 5px;
    /* Firefox 1-3.6 */
    -moz-border-radius: 5px;
    display: inline-block;
    vertical-align: middle;
    text-align: center;
    border: 3px solid #ff9800;
    box-shadow: inset 0px 2px 3px 0px rgba(0, 0, 0, .3), 0px 1px 0px 0px rgba(255, 255, 255, .8);
    -moz-box-shadow: inset 0px 2px 3px 0px rgba(0, 0, 0, .3), 0px 1px 0px 0px rgba(255, 255, 255, .8);
    -webkit-box-shadow: inset 0px 2px 3px 0px rgba(0, 0, 0, .3), 0px 1px 0px 0px rgba(255, 255, 255, .8);
    background-color: #ffffff;
}

input[type=checkbox]:checked:before {
    background-color: Green;
    font-size: 15px;
}

Please advise. Thank you.