Styling browser dialog boxes

(from failblog)

Not only the background's been changed but also the buttons. How can this be done?

That's a screen shot of someone's dialog box.

That person obviously has set a custom theme on their OS with that style. This has not been controlled by the browser or anything that could have been done via the browser.

You can look at the jqueryui dialog box here:

http://jqueryui.com/demos/dialog/

The individual with the browser is probably running a different scheme via the OS which is modifying the dialog box.