Almost —
Take a look at my personal blog. See the “Browser Check” widget? It’s just a text widget with some HTML dumped in. I inserted a <div>in the text box and styled it to have a greyish-blue background. The actual code looks like this:
<div style="font-size:11px; text-align:center; background:#EBEBFA; padding: 6px; margin: -5px -12px -10px -12px;
-moz-border-radius-bottomleft: 12px;
-webkit-border-bottom-left-radius: 12px;
border-bottom-left-radius: 12px;
-moz-border-radius-bottomright: 12px;
-webkit-border-bottom-right-radius: 12px;
border-bottom-right-radius: 12px;">
If you're not using <a href="http://www.mozilla.com/firefox" target="blank">Firefox</a> then you're not experiencing all that this website has to offer.
<br><br>Get <a href="http://www.mozilla.com/firefox" target="blank">Firefox</a> today!
</div>
The negative values for the margin pushes the grey <div> out to the edge. I also made sure that this <div> has the same rounded corners that I apply to all my widgets.
Good luck,
Ken