I am no expert, but you should be able to do it with CSS. I don't know the exact code, but should you be able to find it no problem with a search engine.

This can be done with css. Just add the css code to the stylesheet of your site.

Find the css that corresponds to the table and add:

border-radius: 15px;
-moz-border-radius: 15px;
-webkit-border-radius: 15px;

That will give you nice rounder corners on your table. You can change the 15px to any value smaller or larger to increase or decrease how rounded the corner is.