how do I make rounded borders around everything in a drupal 7 site?
Not every browser has border-radius implemented yet. Use vendor prefixes for those.

.box_round {
     -moz-border-radius: 12px; /* FF1-3.6 */
  -webkit-border-radius: 12px; /* Saf3-4, iOS 1-3.2, Android &lt;1.6 */
          border-radius: 12px; /* Opera 10.5, IE9, Saf5, Chrome, FF4, iOS 4, Android 2.1+ */

  /* useful if you don't want a bg color from leaking outside the border: */        
  -moz-background-clip: padding; -webkit-background-clip: padding-box; background-clip: padding-box; 
}


Snippet from: http://css3please.com/