From what I can see, the problem is probably that the new divs you added in the CSS don't exist in the template. The simplebox HTML code is only
<div>content goes here </div>
<div class="rounded"><div><div><div> Content goes here </div></div></div></div>
shown on the sitepoint page, so the extra divs to be styled with your corner images are missing.
surrounding the div or table you want to add the custom corners and borders to.
In wikitext there's more freedom because you aren't limited to the divs provided in the templates. So you could use the sitepoint method by constructing the box with DIV plugin tags.