You are missing something. This line of code
$("#MainRegion div .resultPanel").addClass("success");
returns
#MainRegion div .resultPanel.success
<div class="resultPanel success"></div>
and not
#MainRegion div .resultPanel .success
<div class="resultPanel">
<div class="success"></div>
</div>
So depending on what you want, you can change your css or the js to target the correct element
if your css is
#MainRegion div .resultPanel.success { background-color: green; }
then your js will work as expected