First, you need to understand that an ASP.NET controls translate into HTML element; and in your case the elements
<table>
,
<tr>
,
<th>
and
<td>
will be used, so you need to design and use appropriate styles for such elements.
Please see my recent answer with a working code sample:
Round the edge of a table[
^].
First of all, pay attention for this wonderful rounding tool, a rounded corner generator:
http://cssround.com/[
^].
—SA