#unchecked
<div id="uniform-undefined" class="checker">
    <span>
        <input type="checkbox" name="18" style="opacity: 0;">
    </span>
</div>

# checked
<div id="uniform-undefined" class="checker">
    <span class="checked">
        <input type="checkbox" name="18" style="opacity: 0;">
    </span>
</div>