I've just finished to upload a series of old notes on CSS development which include a demo showing how to build an alert box with CSS. This demo makes use of CSS system colors to create a platform/OS-dependent layout for the alert box.
The main problem encountered here is that not all browsers support system colors properly (for example, it doesn't work in Chrome). Further, system colors have been deprecated in CSS3 so for future compatibility you should avoid this practice. Instead, you can use JavaScript platform detection to provide the proper layout to each OS (yes, the CSS way it's a lot neater, but still).