If you have
<div>
<input type="checkbox" class="check-with-label" id="idinput" />
<label class="label-for-check" for="idinput">My Label</label>
</div>
you can do
.check-with-label:checked + .label-for-check {
font-weight: bold;
}
See this working. Note that this won't work in non-modern browsers.