Since release 2.0 for Moodle 2.6 I removed the rounded corners of the main container. It was just a change in the design. I got tired of boxes, rounded corners and shadows, that is why I was trying to avoid some of them.
But some people like it, because of that I am going to explain the easiest way to bring them back. The are two ways to do it.
The first one is using LESS and modifying the core CSS stylesheet, it could be easier if you have some experience with LESS. The other option is just using the setting page options of Archaius. I am going to explain the second one.
You just have to go to the settings page and add to the custom CSS the following rule:
-webkit-box-shadow: 2px 4px 8px #888;
-moz-box-shadow: 2px 4px 8px #888;
box-shadow: 2px 4px 8px #888;
Finally, you should change the background color of your website. You can do it using the Background color option in setting page. #f5f5f5 used to be the default color of archaius background when I was using rounded corners.
Remember that you have to clear your moodle cache or put your website in design mode to see your change reflected.
Now you suppose to have rounder corners in your Moodle again.