$(document).ready(function() {
$('input#offer').change(function () {
    if ($(this).attr("checked")) {
        //do the stuff that you would do when 'checked'
        $('p').addClass('bold');
        return;
    }
    //Here do the stuff you want to do when 'unchecked'
    $('p').removeClass('bold');   
});

Toggle Checkbox function

jquery

jQuery