When creating a theme, it is often very handy to break up the CSS into multiple files for the purpose of keeping things better organized. Luckily, Drupal has built-in CSS optimization on the admin/settings/performance page, but during development it is sometime quite easy to run up a large number of CSS files being used.
Getting quick and easy control of rounded corners is a bit of a holy grail for theme developers and CSS jockeys. In order to do rounded corners correctly and have them appear consistent across a wide range of browsers often entails several additional HTML elements (usually DIVs) and several hours of CSS head-banging.
An easy way to get around this, providing you're willing to give up rounded corners in IE (a big "if", I know) is to utilize a couple of browser-specific CSS properties. Both Firefox and Safari utilize the "border-radius" CSS properties. For example: